How can I read a single character from a file? from the keyboard?

You can use the builtin getc function for most filehandles, but it won't (easily) work on a terminal device. For STDIN, either use the Term::ReadKey module from CPAN, or use the sample code in getc.

If your system supports POSIX, you can use the following code, which you'll note turns off echo processing as well. 

    #!/usr/bin/perl -w
    use strict;
    $| = 1;
    for (1..4) {
	my $got;
	print "gimme: ";
	$got = getone();
	print "--> $got\n";
    }
    exit;


    BEGIN {
	use POSIX qw(:termios_h);


	my ($term, $oterm, $echo, $noecho, $fd_stdin);


	$fd_stdin = fileno(STDIN);


	$term     = POSIX::Termios->new();
	$term->getattr($fd_stdin);
	$oterm     = $term->getlflag();


	$echo     = ECHO | ECHOK | ICANON;
	$noecho   = $oterm & ~$echo;


	sub cbreak {
	    $term->setlflag($noecho);
	    $term->setcc(VTIME, 1);
	    $term->setattr($fd_stdin, TCSANOW);
	}


	sub cooked {
	    $term->setlflag($oterm);
	    $term->setcc(VTIME, 0);
	    $term->setattr($fd_stdin, TCSANOW);
	}


	sub getone {
	    my $key = '';
	    cbreak();
	    sysread(STDIN, $key, 1);
	    cooked();
	    return $key;
	}


    }


    END { cooked() }

The Term::ReadKey module from CPAN may be easier to use: 

    use Term::ReadKey;
    open(TTY, "</dev/tty");
    print "Gimme a char: ";
    ReadMode "raw";
    $key = ReadKey 0, *TTY;
    ReadMode "normal";
    printf "\nYou said %s, char number %03d\n",
        $key, ord $key;

For DOS systems, Dan Carson reports the following:

To put the PC in ``raw'' mode, use ioctl with some magic numbers gleaned from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes across the net every so often): 

    $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
    $old_ioctl &= 0xff;
    ioctl(STDIN,1,$old_ioctl | 32);    # Writes it back, setting bit 5

Then to read a single character: 

    sysread(STDIN,$c,1);               # Read a single character

And to put the PC back to ``cooked'' mode: 

    ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.

So now you have $c. If ord == 0, you have a two byte code, which means you hit a special key. Read another byte with sysread, and that value tells you what combination it was according to this table: 

    # PC 2-byte keycodes = ^@ + the following:


    # HEX     KEYS
    # ---     ----
    # 0F      SHF TAB
    # 10-19   ALT QWERTYUIOP
    # 1E-26   ALT ASDFGHJKL
    # 2C-32   ALT ZXCVBNM
    # 3B-44   F1-F10
    # 47-49   HOME,UP,PgUp
    # 4B      LEFT
    # 4D      RIGHT
    # 4F-53   END,DOWN,PgDn,Ins,Del
    # 54-5D   SHF F1-F10
    # 5E-67   CTR F1-F10
    # 68-71   ALT F1-F10
    # 73-77   CTR LEFT,RIGHT,END,PgDn,HOME
    # 78-83   ALT 1234567890-=
    # 84      CTR PgUp

This is all trial and error I did a long time ago, I hope I'm reading the file that worked.