substr odd behavior with list arguments

wfischer has asked for the wisdom of the Perl Monks concerning the following question:

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Re: substr odd behavior with list arguments by choroba (Cardinal) on Mar 22, 2021 at 16:13 UTC
That's because `prototype(\&CORE::substr)` is `$$;$$`. It means each argument is evaluated in scalar context, so in the second case @a is interpreted as `scalar @a`, i.e. the size of the array, which is 2. Use `my ($offset, $length) = a_subroutine_call_that_returns_offset_and_leng +th(@some_args); substr('abcdefg', $offset, $length);` [download] `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re^2: substr odd behavior with list arguments by wfischer (Novice) on Mar 22, 2021 at 16:26 UTC
That's the "why" -- thanks for that clear explanation. (And, yes, that's the workaround I use.)	[reply]
Re: substr odd behavior with list arguments by haukex (Archbishop) on Mar 22, 2021 at 16:19 UTC
I care because I'd like to do this: `substr($string,a_subroutine_call_that_returns_offset_and_length(@some_args))` You can get Perl to ignore the prototype (that was explained by choroba) on Perl 5.16 and up via: `&CORE::substr("abcdefg",@a)`	[reply] [d/l] [select]
Re^2: substr odd behavior with list arguments by LanX (Saint) on Mar 22, 2021 at 16:33 UTC
well if he needs this more often in his code, he could also override° `substr` with `use` `subs` in order to work with other prototypes. But I'd rather write a `my_substr()` sub to avoid confusion. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery} update: °) see https://perldoc.perl.org/perlsub#Overriding-Built-in-Functions	[reply] [d/l] [select]
Re^2: substr odd behavior with list arguments by wfischer (Novice) on Mar 22, 2021 at 16:29 UTC
... And that's the "how," to make Perl act the way I want. Maybe I won't, though -- it might not be prudent to change the prototype on this low-level call. Thanks!	[reply]
Re^3: substr odd behavior with list arguments by haukex (Archbishop) on Mar 22, 2021 at 16:46 UTC
it might not be prudent to change the prototype on this low-level call Note you're not changing it, just telling Perl to ignore it for that one call, meaning you get Perl's standard behavior of flattening the argument list. But indeed it means that you need to be certain that your subroutine will always return the offset and length; the workaround that choroba showed at least gives you the oppertunity to check that. And LanX's suggestion to wrap it in a `sub` is also good.	[reply] [d/l]