as a one-liner:

$ perl -Mstrict -w -e'while(1){ chomp( my $i = <STDIN>); $i eq "yes" &
+& print "yes" && last; $i eq "no" && print "no" && last }'
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as legible code:

#!/usr/bin/perl -w
use strict;
$|++;


while(1)
{
    chomp( my $i = <STDIN> );
    if( $i eq 'yes' ) { print "Yes entered for system.\n"; last }
    if( $i eq 'no' ) { print "No entered for system.\n"; last }
}
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it's an infinite loop--the only way to exit is to enter a valid response.

aw, heck, there's more than one way to do it...

my $response = qr/^(yes|no)$/;
while( chomp( $_ = <STDIN> ) )
{
    /$response/ and print "$1 entered for system.$/" and last
}
[download]

~Particle *accelerates*