#!/usr/bin/perl

use strict;
use warnings;


my $words = "this is a series of words";

$words =~ s{((\w+(?:'\w+)*))}{ my( $res, $y ); ( ( $res, $y ) = &link( $1 ) ) ? "$res $y" : $1 }ge;

print "$words\n";

sub link {
  my $word = shift;
  if( $word eq 'series' ){
    return( ( ucfirst $word ), $word );
  }
  return;
}