#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

my $string = "This     is        a string with   variable     numbers   of    spaces.";
say "original: $string";
my $number_of_substitutions = $string =~ s|\s{2,}| |g;
say "cleaned:  $string";
say "# of substitutions: $number_of_substitutions";

__END__
original: This     is        a string with   variable     numbers   of    spaces.
cleaned:  This is a string with variable numbers of spaces.
# of substitutions: 6