Firstly, Term::Readkey is a pretty small module so it's no great overhead to install and use it and it seemed to do everything right whenever I used it. My advice would be to go with that.

However, if you are unable to do that you probably have to go with changing terminal settings by hand. I copied a recipe out of the 2nd edition Camel Book a long, long time ago and I haven't used it in years. It may give you a start though so here it is.

# RawTerm.pm
#
# Adapted from "Programming PERL", Wall/Christiansen/Schwartz, 2nd Edn
+. pp.474
#
package RawTerm;
use Exporter;
@ISA = ('Exporter');
@EXPORT = ('getone');

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();
}

1;
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If I remember correctly you just call the getone() subroutine to get a single character without needing the <Enter> key. I hope this is of use.

Cheers,

JohnGG