Use select. Example in the while loop below:

#!/usr/local/bin/perl -w
use strict;

BEGIN {
        # from Camel book 2nd ed. p.474                     
        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);
        }
}
END {
        cooked();
}

cbreak();

while (1) {
        my ($rin, $rout, $nfound);
        $rin = '';
        vec($rin, fileno(STDIN), 1)  = 1;

        if ($nfound = select($rout=$rin, undef, undef, 0.001)) {
                my ($key);
                sysread(STDIN, $key, 1);

                if ($key eq 'j') {
                        print "It was j!\n";
                } elsif ($key eq 'q') {
                        print "I'm melting!\n";
                        exit;
                }
        }
}
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