use strict;
use warnings;

my $str = 'C:\fldr1\fld2\fldr3\fldr4\fldr5\test.txt@@\main\Msometest\test12.1x\1';
my ($str2) = $str =~ /(fldr3\\fldr4\\fldr5\\test.txt)/;
print "str2 = $str2\n";
__END__
str2 = fldr3\fldr4\fldr5\test.txt