Note that Perls integers use only 32 (or sometimes 64) bits, and they are signed, so every number larger than 2^31 (or 2^63) is silently converted to a floating point number, loosing some precision when the number grows big:

perl -e "print 1234567890123456789012345678900 % 2"
0
perl -e "print 1234567890123456789012345678901 % 2"
0
[download]

Math::BigInt can help:

perl -MMath::BigInt -e "print Math::BigInt->new('123456789012345678901
+2345678900') % 2"
0
perl -MMath::BigInt -e "print Math::BigInt->new('123456789012345678901
+2345678901') % 2"
1
[download]

Alexander