$url = q|$itemID|;

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

my $itemID = 'something-12345.html';
my $url =  q|$itemID|;
$url =~ /(\d{4,5})\.htm/i;
print "Item ID: $itemID\n";
print "    URL: $url\n";
my $num = $1;
print "    NUM: $1\n";
exit;

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

my $itemID = 'something-12345.html';
my $url =  qq|$itemID|;
$url =~ /(\d{4,5})\.htm/i;
print "Item ID: $itemID\n";
print "    URL: $url\n";
my $num = $1;
print "    NUM: $1\n";
exit;

##</code><code>##

perl -MYAPE::Regex::Explain -E 'say YAPE::Regex::Explain->new("(\d{4})+\.htm")->explain();'

##</code><code>##

The regular expression:

(?-imsx:(d{4})+.htm)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1 (1 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    d{4}                     'd' (4 times)
----------------------------------------------------------------------
  )+                       end of \1 (NOTE: because you are using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  htm                      'htm'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

##</code><code>##

perl -MYAPE::Regex::Explain -E 'say YAPE::Regex::Explain->new("(\d{4,5})\.htm")->explain();'

##</code><code>##

The regular expression:

(?-imsx:(d{4,5}).htm)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    d{4,5}                   'd' (between 4 and 5 times (matching the
                             most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  htm                      'htm'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------