my %prefixes = ( '04025' = [ 'Region1, Region2, Region3' ],
                 ...
               );

##</code><code>##

my ($pref5, $pref4, $pref3, $pref2) = map { substr( $phone, 0, $_ ) }
                                          (5, 4, 3, 2);

my $prefix_length = exists $prefixes{$pref5} ? 5
                  : exists $prefixes{$pref4} ? 4
                  : exists $prefixes{$pref3} ? 3
                  : exists $prefixes{$pref2} ? 2
                  :                            0
                  ;

my $formatted_phone = join( ' ',
                            substr( $phone, 0, $prefix_length),
                            substr( $phone, $prefix_length),
                      );