Here is your problem: p=>1 specifies a precision of 1. See these for 
comparisation:

  $ perl -Mbignum=p,1 -wle '$i = 1; print $i + 123456788,"\n"'
  123456790

  $ perl -Mbignum=p,2 -wle '$i = 1; print $i + 123456788,"\n"'
  123456800

  $ perl -Mbignum=p,5 -wle '$i = 1; print $i + 123456788,"\n"'
  123500000

Looks to me it works as designed.

Here is the same with different accuracy values:

  $ perl -Mbignum=a,1 -wle '$i = 1; print $i + 123456788,"\n"'
  100000000

  $ perl -Mbignum=a,2 -wle '$i = 1; print $i + 123456788,"\n"'
  120000000

  $ perl -Mbignum=a,3 -wle '$i = 1; print $i + 123456788,"\n"'
  123000000

and with negative values of p:

  $ perl -Mbignum=p,-3 -wle '$i = 1; print $i + 123456789.1,"\n"'
  123456790.100

  $ perl -Mbignum=p,-6 -wle '$i = 1; print $i + 123456789.1,"\n"'
  123456790.100000

Best regards,

Tels