Considering that sprintf knows how to do rounding, it's just a trivial matter of scaling: 
#!/usr/bin/perl

use strict;
use warnings;

foreach my $n (0, 1, 10, 24, 25, 26, 49, 50, 51, 74, 75, 76) {
    printf "%3d: %3d\n", $n, 50 * sprintf "%.0f" => $n / 50;
}

__END__
  0:   0
  1:   0
 10:   0
 24:   0
 25:   0
 26:  50
 49:  50
 50:  50
 51:  50
 74:  50
 75: 100
 76: 100

[download]
Note the bankers rounding.

Abigail

In reply to Re: Rounding to the nearest 50 by Abigail-II
in thread Rounding to the nearest 50 by Hammy

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