use strict;
use warnings;
use Data::Dumper;

my $post = "Hi, erm does anyone know how to do a string replacement that caters for all the following scenario? 500 changes to 000 or 0 988 changes to 000 or 0 1210 changes to 1000 1300 changes to 1000 2134 changes to 2000 2500 changes to 2000 Thanks in advance.";

my %replacement = $post =~ /(\d+) changes to (\d+)/g;

print Dumper \%replacement;