my $endung='.txt';
my $fname="/das/ist/eine/datei.txt";

my $e_length=length($endung);

# substring
print "SUBSTR OK\n" if(substr($fname,-$e_length,$e_length) eq $endung);

# rindex
print "RINDEX OK\n" if(rindex($fname,$endung)==length($fname)-$e_length);

# index
print "INDEX  OK\n" if(index($fname,$endung,length($fname)-$e_length)==length($fname)-$e_length);

#regexp
print "REGEXP OK\n" if($fname=~/\Q$endung\E$/);