%mapping seems to be a HoH in the first cycle, and a HoA in the second. I'd probably modify to (untested):
my %mapping;
while (<>) {
  my ($key, $value) = /^(.*?)=(.*)$/ or die "Cannot parse $_";
  push @{$mapping{$key}}, split /, /, $value;
}
for my $word (sort keys %mapping) {
  my @aliases = sort @{$mapping{$word}};
  print "$word=", join(", ", @aliases), "\n";
}

[download]
or, if scared by possible repetitions (untested, again):
my %mapping;
while (<>) {
  my ($key, $value) = /^(.*?)=(.*)$/ or die "Cannot parse $_";
  $mapping{$key}{$_} = 1 for split /, /, $value;
}
for my $word (sort keys %mapping) {
  my @aliases = sort keys %{$mapping{$word}};
  print "$word=", join(", ", @aliases), "\n";
}

[download]

Flavio
perl -ple'$_=reverse' <<<ti.xittelop@oivalf

Don't fool yourself.

In reply to Re^2: Remove duplicate words from a dictionary by polettix
in thread Remove duplicate words from a dictionary by 1Nf3

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