use strict;
use warnings;

for (qw/cat dog catfoo catdog/) {
    print $_ =~ /^cat(?!dog)/ ? '[X]' : '[ ]',
          "$_\n";
}
__END__
[X]cat
[ ]dog
[X]catfoo
[ ]catdog

##</code><code>##

use strict;
use warnings;

for (qw/cat dog catfoo catdog catdogs catdogcollie/) {
    print $_ =~ /^cat(?!dog(?!s|collie))/ ? '[X]' : '[ ]',
          "$_\n";
}
__END__
[X]cat
[ ]dog
[X]catfoo
[ ]catdog
[X]catdogs
[X]catdogcollie