Like I said in the CB,

my $base = 'C:\\fldr1\\fld2\\';
$str =~ s/\@\@.*//s;
$str =~ s/^\Q$base\E//;
[download]

or better yet

use File::Spec::Functions qw( abs2rel );
my $base = 'C:\\fldr1\\fld2';
$str =~ s/\@\@.*//s;
$str = abs2rel($str, $base);
[download]