See tye's References Quick Reference for how to dereference things:

use strict;
use Data::Dumper;
my $idx = 1;

my %foo = (
    bar => [1, 2, 3, ],
);
print "foo before: " . Dumper \%foo;

my $bar = 'bar';
my @deleted = splice @{ $foo{ $bar }}, $idx, 1;

print "foo after: " . Dumper \%foo;
print "Deleted: " . Dumper \@deleted;

__END__

foo before: $VAR1 = {
          'bar' => [
                     1,
                     2,
                     3
                   ]
        };
foo after: $VAR1 = {
          'bar' => [
                     1,
                     3
                   ]
        };
Deleted: $VAR1 = [
          2
        ];
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