use strict;
use warnings;

my $string = 'insertreallylongstringwithlotsandlotsoffunkycharactersandifonefeelsreallyspecialperhapsevenaspacesomewhere";
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++;
  }