For what you're trying to do, a simple regular expression match will work. If you just want to pull out the sequence of numbers from $oldValue, then try something like:

if ( $oldValue =~ m|(\d+)| ) {
   $newValue = $1;
}
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For what you're trying to do with split, you need to specify which list element you want to put into $newValue. Since the list starts at element 0, you'll want element 1. You can access it with the following statement:

$newValue=(split("_",$oldValue,2))[1]
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Note, this will return everything after the underscore, and not just the numbers.

$oldValue =~ /_(\d+)\./;
$newValue = $1;
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It's a good idea to always check if a regex succeeds before using $1 and friends, or you may get a result from a previous match. One way to do this implicitly: ($newValue) = $oldValue =~ /_(\d+)\./; This leaves $newValue as undef on failure (because the match returns an empty list). You can even set newValue and check the result all at once: ($newValue) = $oldValue =~ /_(\d+)\./ or warn "bad format: '$oldValue'"; or

if (($newValue) = $oldValue =~ /_(\d+)\./) {
   # all is ok
} else {
   # something went wrong
}
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Your question is not very clear. All of the solutions offered above assume, more or less, that you want to remove any non-numeric characters from your string. However, if what you really want is to remove everything up to and including the underscore, regardless of whether it's numeric or whatever, you could do this:

use strict;
use warnings;
my $old_value = 'UPNOON33_A303.doc';
my $new_value = (split /_/, $old_value)[-1];
print $new_value;
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hth

dave