Why chop? Do you really hate substr so much??

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

print "What string would you like to twaddle? ";
chomp(my $twiddle = <STDIN>);
$l = length $twiddle;
# Up...
print substr($twiddle,0,$_),"\n" for (1..$l);
# Down...
print substr($twiddle,0,-$_),"\n" for (1-$l..-1);
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(I also took the liberty of removing the blank line at the end, which I assumed unintentional)