while ($line = <READTHIS>) {
    if ($line =~ /cat/) {
        print "yeppers!";
    }
}

##</code><code>##

while ($line = <READTHIS>) {
    if ($line =~ /cat/) {
        print "yeppers!";
    }
    else{
        print $line;
    }
}

##</code><code>##

while ($line = <READTHIS>) {
    print "MOO!\n";
    if ($line =~ /cat/) {
        print "yeppers!";
    }
}