One solution using Text::Table:

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

use Text::Table;
use constant CAPACITY => 3;

my @lines = `df -hP`;
my $header = shift @lines;
# Do not create two columns for "Mounted" and "On"
$header =~ s/Mounted /Mounted_/;
my $table = Text::Table->new(split ' ', $header);
{
    no warnings 'numeric'; # Ignore % signs
    @lines = sort { $a->[CAPACITY] <=> $b->[CAPACITY] }
             map [split ' ', $_], @lines;
}
$table->load(@lines);
print $table;
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In my systems, Capacity is called "Use%" and is in a different column.