Use (?i):

use strict;
use warnings;

my @strs = (
    '<a href="http://somelink.com"',
    '<a href="http://SOMELINK.com"',
    '<a HREF="http://somelink.com"',
  );

for my $str (@strs) {
  print "$str matches\n" if $str =~ m|<a href=(?:(?i)"http://somelink.
+com")|;
}
[download]

Prints:

<a href="http://somelink.com" matches
<a href="http://SOMELINK.com" matches
[download]