$ perl whomany.pl 2 675
Global symbol "$arg2" requires explicit package name at test.pl line 2
+0.
Execution of test.pl aborted due to compilation errors.
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So I am guessing that the code you pasted is not the code that's actually running. You don't declare $arg2 and are missing ->new in a few places.

Also, bdiv and bmul modify inplace. I believe what you were trying to write is:

#!/usr/bin/perl -w
use strict;
use Math::BigInt;

#BigInt KiloByte Unit
my $KB = Math::BigInt->new( "1024" );

#BigInt MegaByte Unit
my $MB = $KB->copy()->bmul( $KB );

#BigInt GigaByte Unit
my $GB = $MB->copy()->bmul( $KB );

print "$KB, $MB, $GB\n";
my $arg1 = Math::BigInt->new( shift );
my $arg2 = Math::BigInt->new( shift );

# arg1 is in GBs
my $GBarg1 = $arg1->copy()->bmul( $GB );

# arg2 is in MBs
my $MBarg2 = $arg2->copy()->bmul( $MB );

print $GBarg1, " ", $MBarg2, "\n";
# why rc is 0?
my ($rc, $rem) = $GBarg1->copy()->bdiv( $MBarg2 );
print "rc is [$rc]\n";
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Update: Clean up code a bit