Define "work" because according to my definition, it does ;)

for my $word ( qw[ absorb absorbo absorbs absorbed absorbarararara ]){
    $word =~ /(absorb|absorbs|absorbed)/i
    and warn " $1 ";
}
__END__
 absorb  at - line 2.
 absorb  at - line 2.
 absorb  at - line 2.
 absorb  at - line 2.
 absorb  at - line 2.
[download]

Perhaps you wish to investigate some anchors (namely ^ and $). Also of interest might be (besides the manual pages) YAPE::Regex::Explain.

update: Enlils response reminded me of Regex::PreSuf

use Regex::PreSuf;
my $re = presuf(qw(absorb absorbs absorbed absorbing ));
die $re;
__END__
absorb(?:ed|ing|s)? at - line 3.
[download]