use strict;
use warnings;

my $string = 'insertreallylongstringwithlotsandlotsoffunkycharactersan
+difonefeelsreallyspecialperhapsevenaspacesomewhere";
my $char_to_be_matched = 'X';
my @matches;

my $offset = 0;

foreach my $char (split(//, $string))
  {
  if($char eq $char_to_be_matched) #alternatively some regex magic
    { push(@matches, $offset); }
  $offset++;
  }
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I'm pretty certain there's one or more things that I'm not taking into account in this example, but you get the general idea ;-)