#!/usr/bin/perl -w
use strict;

my %x = ( 1 => [qw(d e f)],
	  2 => [qw(a b c)],
	  3 => [qw(g h i)]
	  );

my %rev = map { my $key=$_; map { $_=>$key} @{$x{$key}} } keys %x;
print "$_ -> $rev{$_}\n" foreach sort keys %rev;