As a former Perl newbie, I'd suggest avoiding string literals as hash keys. One typo can take you hours to find, because the "uninitialized value" line number will be where you try to use it, and not where you typoed.

my $LB = 'LB';
my $LENGTH_RANGES = 'LENGTH_RANGES';
my $WIDTH_RANGES = 'WIDTH_RANGES';
my $EM_POLY = 'EM_POLY';

# ...sometime later...
$metals{$LB}{$LENGTH_RANGES} = [ 0, 10 ];
$metals{$LB}{$WIDTH_RANGES} = [ 0, 1.080 ];
$metals{$LB}{$EM_POLY} = "(-0.665364 + (9.6216 * W))*(10/L)";
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Then Perl will check your spelling on the variable names (which are really constants, but that's another kettle of fish).

But as hdb said, Data::Dumper is invaluable in this area.