I haven't looked at your code in detail yet, nor tried to use the interface you've got already, specifically so that I would not be influenced.

My first thought on how I would like to use a conversions module is that I would pass the source and destination units and it would create a named sub in my package namespace (like use constant does).

In use, it might look something like this:

# if (units) specified after text unit description 
# (which should understand most normal abbrevs.)
# then if the input is a string it is inspected for units,
# and the conversion done in the appropriate direction
# If the input is purely numeric (if ONLY Perl_looks_like_number() was
+ accessible!) 
# then the conversion is in the direction specified by the order of de
+claration time parameters.

use Units::Convert FT_IN_2_MTRS => q[ft(')inches(")  meters(m)]; 

print FT_IN_2_MTRS q{5'10"};     # prints '1.7773m'

print FT_IN_2_MTRS 5.8333;     # prints 1.77773

# No (units) specified on delclaration, input must be numeric, convers
+ion works in 1 direction only.

use Units::Convert MPH_2_KPH => q[mph kph]; 

print MPH_2_KPH 70;                              # prints 112

print MPH_2_KPH '70mph';                     # Causes warn or die 

my @limits = qw(30 40 50 60 70);

print "@{[ MPH_2_KPH @limits ]}";          # prints 50 64 80 96 112

# An extension would be for the user to supply a sprintf-style format 
+string 
# that is used for the formating/precision of the output.
# Once we get string .v. numeric contexts, the sub could determine whe
+n to append the units or not

use Units::Convert CI_2_CC => 'inch^3(%.2f ci) cm^3(%.f cc)';    

print CI_2_CC 500;                                # prints 8183
print CI_2_CC '500 ci';                          # prints '8183 cc'

# If an itermediate conversion is required, this could be specified on
+ the declaration
# I'm not sure this is a good example, but it's the one that came to m
+ind.

use Units::Convert UK_2_METRIC_WEIGHTS => 'stones(st)_pounds(lbs) lbs 
+kilograms(%.1f kilos)';

print UK_2_METRIC_WEIGHTS '11st 7lbs';  # prints '73.2 kilos'

print UK_2_METRIC_WEIGHTS 11.5;  # prints 73.2

print UK_2_METRIC_WEIGHTS '11.5_';  # prints '73.2 kilos' maybe? 
# The presence of an underscore forces output formattting (if supplied
+)?
[download]

Final thought on the precision and under/overflow thing. Perhaps, if a flag is set, the routines could return BigInt/Floats if the standrad precisions will cause accuracy loss? I haven't thought that through, so I don't know what the implications are.

Now I'll read your code and see if I'm completly off-base, but I like to look at things from my own perspective first when I can :^).

If you decide not to go ahead with teh module, let me know and I will.