You say users can't modify this sub, but I take it you can, so you could pass an optional index value to your sub:

sample($t,0);

sub sample {
    my $t = shift;
    my $index = shift;
    my ($p, @r, $v, $rt);
    $t->get_handles(params=>\$p, rtrs=>\@r, vars=>\$v, t=>\$rt);
    
    my @subs;
    if (defined $index) {
        my $rh = $r[$index];
        <some command at execution>
    }
    else {
        foreach my $rh (@r) {
            <some command at execution>
            ...
        }
    }
}
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