Argument list item alias-ing

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

sub m{
 $_[0]=~s///;
}
m("String");
[download]

Will, obviously, not work, as it tries to modify a read-only value.
Same is the case with:

*_=\"Hello";
s///;
[download]

But:

sub m{
 local *_=\$_[0];
 s///;
}
 m("String");
[download]

Will run. Which is weird, as $_ is an alias to $_[0] which is an alias to a readonly value. $_[0] won't change, but $_ will, which is again weird, as $_ should be an alias to it. Can someone clarify what's going on to me?

Comment on Argument list item alias-ing Select or Download Code

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Re: Argument list item alias-ing by broquaint (Abbot) on Apr 16, 2003 at 11:02 UTC
Will run. Which is weird You're actually matching the string as you're using the balanced delimiters syntax of a regex match e.g `shell> perl -MO=Deparse - sub m{ local _=\$_[0]; s///; } m("String"); sub m { local _ = \$_[0]; s///; } /"String"/; - syntax OK` [download] But if you change that sub name to `replace` then you get the expected error message `sub replace { local *_=\$_[0]; s///; } replace("String"); Modification of a read-only value attempted at pmsopw_250834.pl line 3 +.` [download] HTH `_________ broquaint`	[reply] [d/l] [select]
Re: Argument list item alias-ing by Ido (Hermit) on Apr 16, 2003 at 11:11 UTC
Oooops. Wrong subname..Sorry about that... Anyway, here(ActivePerl 5.8.0, Build 805), neither: `sub subname{ local *_=\$_[0]; s///; } subname("String");` [download] Gives any error message. So..is it a bug?A feature?Another stupid mistake of myself?	[reply] [d/l]
Re: Re: Argument list item alias-ing by broquaint (Abbot) on Apr 16, 2003 at 13:40 UTC
Sounds like a bug to me as I get an error message here `sub subname { local _=\$_[0]; s///; } subname("String"); Modification of a read-only value attempted at - line 3.` [download] This error should occurs because you're 'aliasing' `$_` to `$_[0]`, which essentially equates to both pointing to the same scalar value e.g `sub { print "\$_[0]: ",\$_[0], $/; local _= \$_[0]; print "\$_: ",*main::_{SCALAR}, $/; s///; }->("foo"); __output__ $_[0]: SCALAR(0x8107ec8) $_: SCALAR(0x8107ec8) Modification of a read-only value attempted at - line 5.` [download] HTH `_________ broquaint`	[reply] [d/l] [select]
Re: Argument list item alias-ing by nothingmuch (Priest) on Apr 16, 2003 at 12:14 UTC
That shouldn't happen. If you're dealing with possible read only's perhaps you should consider using Scalar::Util::readonly();. The behaviour you have witnessed is, i think, some kind of bug. Any guts guru care to share h(er\|is) thoughts? -nuffin zz zZ Z Z #!perl	[reply]