This problem just cries out for regex in Perl.
This is not a sub-string problem.
sub string is the wrong tool for the job.

#!/usr/bin/perl -w
use strict;

my ($var1) = 'stuff_to_cut_out_with_substr_FOO';
my ($var2) = 'stuff_to_cut_out_with_substr_BAR';
my ($var3) = 'stuff_to_cut_out_with_substr_LOST';

my @paths;

foreach my $line ($var1, $var2, $var3)
{
   push @paths, $line =~ /_([A-Za-z]+)$/; #stuff after last _
}

print join("/",@paths),"\n";

__END__
prints "FOO/BAR/LOST"
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