You could use zip from List::MoreUtils to mesh the arrays into a hash, then sort the keys by the values:

use strict; use warnings; use feature 'say';
use Data::Dumper;

use List::MoreUtils 'zip';

my @x = qw/d b c e a f/;
my @y = qw/4 6 5 2 9 1/;

my %h = zip @x, @y;
my @z = ( sort { $h{ $b } <=> $h{ $a } } keys %h )[0..3];

say Dumper \@z;

__END__
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Output:

$VAR1 = [
          'a',
          'b',
          'c',
          'd'
        ];
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Hope this helps!