You may not have tried a big enough number. 

$ perl5.8.6 -MMath::BigInt -e '$i = Math::BigInt->new("123_456_789_000
+_000_000_000"); printf "%045d\n", $i'
-00000000000000000000000000000000000000000001

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In reply to Re^4: Math::BigInt and Leading Zeros by Tanktalus
in thread Math::BigInt and Leading Zeros by aplonis

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