Hello, don't you think this is enough:

#!/usr/bin/perl

$S = "a ,   b, c,d";

sub modif
{
  my ($S) = @_;
  $S =~ s/(.*),/$1 and /;
  $S =~ s/\s*,\s*/, /g;
  return $S;
}

print "\$S=$S ==> ". modif($S) . "\n";
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in case of an array (or list), join it with commas before:

@Arr = (1 ,   2, 3,4);
print "Array = ". modif(join(',',@Arr)) . "\n";
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I don't think is worth a module! Hope it helps !