That's not equivalent. It fails for negative numbers, as well as positive numbers that requires a multiple of 8 bits to represent (e.g. 128-255, and 32678-65535).

The following addresses the second of the above limitations:

sub bigint_to_bytearray {
   my $bigint = shift;
   die "Negative numbers not supported" if $bigint < 0;

   my @bytes = unpack('c*', $bigint->to_bytes);
   unshift @bytes, 0x00 if $bytes[0] < 0;  # Add sign bit if necessary
   return @bytes;
}
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The above requires a relatively new version of Math::BigInt. The following is less efficient, but it works on far older versions:

sub bigint_to_bytearray {
   my $bigint = shift;
   die "Negative numbers not supported" if $bigint < 0;

   my @bytes = unpack('c*', pack('H*', substr($bigint->as_hex, 2)));
   unshift @bytes, 0x00 if $bytes[0] < 0;  # Add sign bit if necessary
   return @bytes;
}
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