What is the difference of big or small number by calling bignum module?

When my code tested on 2 digit small number it's good to run, but change to real large number it gives out error:

can't return outside a subroutine.....

When I again test the error subroutine individually with real big number, it works good again

The code has no problem otherwise small number won't run. But why it give error on large numbers?

Here is just little subroutine, tested good on itself

use bignum;
$big_number = xxxxxx; #declared a 100 digits integer.
$another_big_number=xxxxxx;# 100 digits.
print operation($another_big_number);
sub operation {
    my $product = eval $_[0];
    return  abs $big_number - $product;
}

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In reply to bignum, can't return outside a subroutine by zli034

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