I would suggest using File::Basename:

use File::Basename;

my $path = 'c:/reports/check/test.txt';
fileparse_set_fstype('MSWin32'); # or is it 'MSDOS'?
my ($name, $base, $suffix) = fileparse($path, '.txt');
$name = 'design.' . $name;

print join '', $base, $name, $suffix, "\n";
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