use File::Spec;
my $goners = 3; # how many?
my $str    = 'C:\fldr1\fld2\fldr3\fldr4\fldr5\test.txt@@\main\Msometest\test12.1x\1';
$str       =~ s/\@/\@.*//;
my @dirs   = File::Spec->splitdir($str);
splice @dirs, 0, $goners;
print File::Spec->catdir(@dirs);