my %fields = (
    version1 => {
        1 => 'Field 1',
        2 => 'Field 2',
        3 => 'Field 3',
    },
    version2 => {
        1 => 'Field 1',
        2 => 'Field 2',
        3 => 'Field 3',
        4 => 'Field 4',
    },
    version3 => {
        1 => 'Field 1',
        2 => 'Field 2',
        3 => 'Field 3',
        4 => 'Field 4',
        5 => 'Field 5',
    },
);

my %synonyms = (
    version4 => 'version3',
    # ... maybe you want others? ...
);

$fields{$_} = $fields{ $synonyms{$_} } for ( keys %synonyms );