#!/usr/bin/perl -w
use strict;
my $string = "stash_ST1.2.3.4_1.ROLLBACK.check.20170228-101051.435944.txt";
my $action = '';
my @parts = split(/\./, $string); # split on dot
for (0..$#parts){
    # slot after ROLLBACK
    $action = $parts[$_+1] if $parts[$_] eq "ROLLBACK";
}    
print "$action\n";