use strict;
use warnings;

foreach my $test ('..', 'file.txt', 'blah.abc.txt')
{
   my ($suffix) = $test =~ /(\.[^.]+)$/;
   $suffix //= '';  #suffix is null string if no match
   print "test=$test  suffix=$suffix\n";
}

__END__
test=..  suffix=
test=file.txt  suffix=.txt
test=blah.abc.txt  suffix=.txt