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

#this gets chars until first digit...
my $search = "some_string200912.txt";
my $alpha1 = ($search =~ m/(.*?)\d+.txt/)[0];

#this gets chars until first digit...
my $to_modify = "some_value200912.txt"; 
my $alpha2 = ($to_modify =~ m/(.*?)\d+.txt/)[0];

#this modifies (assigns new value to $to_modify)
$to_modify = "$alpha2"."_"."$alpha1";
print "$to_modify\n";

__END__
prints:
some_value_some_string

##</code><code>##

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

#this gets chars until first digit...
my $search = "some_string200912.txt";
(my $alpha1) = $search =~ m/(.*?)\d+.txt/;

#this gets chars until first digit...
my $to_modify = "some_value200912.txt"; 
(my $alpha2) = $to_modify =~ m/(.*?)\d+.txt/;

#this modifies (assigns new value to $to_modify)
$to_modify = "$alpha2"."_"."$alpha1";
print "$to_modify\n";

__END__
prints:
some_value_some_string