It probably is not possible to predict the results in every case. Consider these two runs of your function. Only the second replicates your original problem.

use strict;
use warnings;
no warnings 'experimental::signatures';
use feature 'signatures';

sub amnt($amn)
{
my $amount=$_[0];
print "$amount i m checking amount before padding,\n";
my $padamnt = sprintf("%016d",$amount);
print "$padamnt i m checking amount after padding,\n";
 return $padamnt;
}

print "EXACT integer\n";
amnt(488715);

print "\n\nNot quite an integer\n";
amnt(488715 - 4e-10);
[download]

OUTPUT:

EXACT integer
488715 i m checking amount before padding,
0000000000488715 i m checking amount after padding,


Not quite an integer
488715 i m checking amount before padding,
0000000000488714 i m checking amount after padding,
[download]