You could use a function like this:

#!/usr/bin/perl
use strict;
use warnings;

sub cut_fill ($$);


my $word   = 'someword';


print "'", cut_fill($word,  5), "'\n";
print "'", cut_fill($word, 10), "'\n";


sub cut_fill ($$) {
    my ($word, $length) = @_;
    if (length($word) >= $length) {
        $word = substr($word, 0, $length);
    }
    else {
        $word .= ' 'x($length - length($word));
    }
    return $word;
}
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Update: Or you can use sprintf of corse, as shown in the solutions below. But if you need something more special, a function like this will be handy.