#! perl
use strict;
use warnings;
use Tie::File;

my $filename = 'data.txt';

tie  my @lines, 'Tie::File', $filename or die "Cannot tie file '$filename' to array: $!";

for (my $i = 0; $i < @lines; ++$i)
{
    my @fields  = split ' ', $lines[$i];
    $fields[1] .= ' ' x (50 - length $fields[1]);
    $lines[$i]  = join(' ', @fields);
}

untie @lines;