my $pid = open KID,  '-|', 
    'pmemd -i mdin -c inpcrd -p prmtop -r restrt -o con -x mdcrd' 
    or die $!;

open OUT, '>', 'mdout' or die $!;

while( <KID> ) {
    kill 'INT', $pid if m[some data];
    print OUT;
}

##</code><code>##

my $pid = open KID,  '-|', 
    qq[pmemd -i mdin -c inpcrd -p prmtop -r restrt -o /dev/tty -x mdcrd] 
    or die $!;

open OUT, '>', 'mdout' or die $!;

while( <KID> ) {
    kill 'INT', $pid if m[some data];
    print OUT;
}