Here is a way you could encapsulate the chaining in a subroutine

use strict;
my $str = "the boy walked the dog";
$str=ChainSubs($str,
           walked=>'fed',
           boy=>'girl',
           dog=>'Audrey II'
          );
print $str;

sub ChainSubs{
  my ($string,%subs)=@_;
  foreach my $original (keys %subs) {
    $string=~s/$original/$subs{$original}/;
  }
  $string;
}
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This of course assumes that you don't care what order the substitutions are done in. If you did care, you could walk through @_ two elements at a time like this:

sub ChainSubs{
  my $string = shift;
  for (my $i=0;$i<@_;$i+=2) {
    $string=~s/$_[$i]/$_[$i+1]/g;
  }
  return $string;
}
[download]