How does @filesize_names get altered by the map when the conditional in make_singular is true? When the conditional is false, @filesize_names is not altered by the map outside make_singular.

#!/usr/bin/perl
use strict;
use warnings FATAL => qw( all );

use Data::Dumper;
use List::MoreUtils qw(firstidx);

use lib 'lib';
use Base::Nifty qw(pretty_number);

local $\ = "\n";

my @filesize_names = qw(bit nibble byte kilobyte megabyte gigabyte ter
+abyte petabyte exabyte zettabyte yottabyte);

# I put the these sizes here just for my information.
my %little_filesizes;
$little_filesizes{bit} = 1;
$little_filesizes{nibble} = $little_filesizes{bit} * 4;
$little_filesizes{byte} = $little_filesizes{bit} * 8;

# I never know when I'll want a random file size.
sub random_filesize {
  return $filesize_names[rand @filesize_names]
}

sub make_singular {
  my $word = shift;
  $word =~ s/s$//;
  
  my @short_sizes = map(s/^(\w)\w{1,}$/$1b/,@filesize_names);
  if (grep(/\L$word\E/,@short_sizes)) {
    $word = $filesize_names[firstidx { $_ eq lc $word} @short_sizes];
  }
  
  return lc $word;
}

# from tye: my %hash; @hash{@sizes} = 0..$#sizes;
# from MidLifeXis: $result = $original * $units{$inmultiplier} / $unit
+s{$outmultiplier}

sub convert_filesize {
  my %opt = @_;

  # I took out bits and nibbles just to keep me sane.
  my @filesizes = grep($_ =~ /byte/,@filesize_names);
  
  my $from = firstidx { $_ eq make_singular($opt{from}) } @filesizes;
  my $to   = firstidx { $_ eq make_singular($opt{to}) } @filesizes;
  my $dec  = $opt{decimals} ? $opt{decimals} : 0;
  my $base = $opt{base} ? $opt{base} : 1024;
  
  my ($diff,$converted);
  
  if ( $from > $to ) {
    $diff = $from - $to;
    $converted = $opt{size} * ($base ** $diff);
  }
  elsif ( $to > $from ) {
    $diff = $to - $from;
    $converted = $opt{size} / ($base ** $diff);
  }
  else {
    $converted = $opt{size};
  }
  
  my $org_filesize = pretty_number($dec,$opt{size});
  my $new_filesize = pretty_number($dec,$converted);
  return "$org_filesize $opt{from} ($from) is $new_filesize $opt{to} (
+$to)";
}

print random_filesize;

my $conversion = convert_filesize(
  size => 10000000,
  from => 'bytes',
  to => 'megabytes',
);
print $conversion;
[download]

I just tried...

sub make_singular {
  my $word = shift;
  $word =~ s/s$//;
  
  my @in_sub_filesize_names = @filesize_names; # added this line, the 
+output is still wrong.
  my @short_sizes = map(s/^(\w)\w{1,}$/$1b/,@in_sub_filesize_names);
  if (grep(/\L$word\E/,@short_sizes)) {
    $word = $filesize_names[firstidx { $_ eq lc $word} @short_sizes];
  }
  
  return lc $word;
}
[download]