Use File::Basename, it will work well under almost any OS. The docs are quite clear ;)

Update: ok, an example:

#!/usr/bin/perl
use strict;
use warnings;
use File::Basename qw( fileparse );
 
my $tempvalue = 'c:/reports/check/test.txt';
my ($name, $path) = fileparse($tempvalue);
my $designvalue = $path . "design." . $name;
print "'$tempvalue' => '$designvalue'\n";
 
__END__
 
'c:/reports/check/test.txt' => 'c:/reports/check/design.test.txt'
[download]