The interface I would want would be something like:     my @new= ConvertTo( $toUnits, $fromUnits, @old ); which would convert the numbers in @old from $fromUnits to $toUnits and return the results. So:

#   <--------- inches ---------><----- ft ----->
    my @inches= ConvertTo( 'in', 'ft', 1, 2, 3 );
    # @inches is now ( 12, 24, 36 )
[download]

Note that it is "to" followed by "from" so the unit name is closest to the numbers that are/will be in those units.

If I want to do a lot of conversions but not all at once, then:

    my $toInFromFt= ConvertTo( 'in', 'ft' );
    while(  <IN>  ) {
        chomp;
        print OUT $toInFromFt->($_), $/;
    }
[download]

And I'd probably support aliases for units and include a "long alias" for every unit: "inch", "feet", "meter", "centimeter", "Centigrade", "Farenheit", "Kelvin", "second", "hour", etc. just to avoid confusion.

I'd probably put the unit data after __DATA__ so you could just append more units to be able to support them.

The "all possible conversions" is interesting for interactive exploration, but I don't see nearly as much use for it as requesting a specific conversion.

For finding the conversion, I'd look for a direct conversion, but if there isn't one, form the list of all possible conversions and then repeat for each of those:

my @table = (
    #### temperature
    [ c => f =>
        sub { ($_[0] * 9/5) + 32 },
        sub { ($_[0] - 32) * 5/9 },
    ],
    [ c => k =>
        sub { $_[0] + 273.16 },
        sub { $_[0] - 273.16 },
    ],

    #### mass/weight
    [ kg   =>  g =>   1000 ],
    [ floz =>  g =>     30 ],
    [ lbs  => kg => 0.4536 ],

    #### distance
    [ ft   => m  =>   0.3048 ],
    [ ft   => in =>       12 ],
    [ in   => cm =>     2.54 ],
    [ m    => cm =>      100 ],
    [ yd   => m  =>   0.9144 ],
    [ km   => m  =>     1000 ],
    [ mile => km => 1.609347 ],
);

my %table;
for my $conv (  @table  ) {
    my( $from, $to, $conv, $rev )= @$conv;
    if(  exists $table{$from}{$to}  ) {
        warn "Duplicate conversions to $to from $from.\n";
    }
    $table{$from}{$to}= $conv;
    if(  $rev  ) {
        if(  exists $table{$from}{$to}  ) {
            warn "Duplicate conversions to $from from $to.\n";
        }
        $table{$to}{$from}= $rev;
    }
}
# Handle reverse conversions when a better one isn't provided:
for my $conv (  @table  ) {
    my( $from, $to, $conv )= @$conv;
    if(  ! ref($conv)  &&  ! exists $table{$to}{$from}  ) {
        $table{$to}{$from}=
            sub { $_[0] / $conv };
    }
}

sub FindConversionPathTo {
    my( $dest, $source )= @_;
    my( @sol );
    $source= { $source => "$source " };
    while(  ! @sol  ) {
        for my $src (  keys %sources  ) {
            if(  exists $table{$src}{$dest}  ) {
                $source{$src} .= "$dest ";
                push @sol, $src;
            } else {
                for my $dest (  keys %{$table{$src}  ) {
                    if(  ! exists $source{$dest}  ) {
                        $source{$dest}=
                            $source{$src} . "$dest ";
                    }
                }
            }
        }
    }
    # Pick one of the solutions at random: (:
    return split ' ', (@source{@sol})[rand @sol];
}
[download]

Untested (I hope you don't mind).

        - tye

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