FAQ. See How can I remove duplicate elements from a list or array?

But maybe this will help:

my %name_of_id;

for my $id ( @all_ids )
{
    if ( ! exists $name_of_id{$id} )
    {
        my $name = get_name_of_id($id); # whatever

        $name_of_id{$id} = $name;
    }
}
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This could be more compactly written as

$name_of_id{$_} ||= get_name_of_id($_)
    for @all_ids;
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but only if names are guaranteed to have non-false values.