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local (the keyword) has NOTHING to do with local variables. In Perl, you use my (the keyword) to declare local (i.e., lexically scoped, or 'lexical') variables.

The keyword local localizes a variable, whether it's named or anonymous or global (i.e., package variables, which are in the symbol table). To localize a variable is to stash away its value in such a way that exiting the enclosing scope will result in its being restored. For example:

use vars qw( $x ); # a package global $x = 'outer'; # this block: { my $save_x = $x; # localize $x by saving its value $x = 'inner'; # change $x to hearts content some_sub( $x ); # some_sub() sees $x with value 'inner' $x = $save_x; # restore original value just before exiting b +lock } # is functionally identical to this block: { local $x = 'inner'; some_sub( $x ); } # in both cases, some_sub() is called with the package global $x set # to 'inner' and in both cases, the value of $x is restored when the # block exits, except in the 'local' case, it is not necessary to # explicitly restore $x

One example of its use is with the Perl global, $/. Often, (in a subroutine, for example) it is necessary to set $/ = undef;, but to do so may break code which assumes its default value.

# need to 'slurp' in an entire file { local $/ = undef; # (read the file, etc.) } # $/ is now restored to whatever it was before

Hopefully, this explains WHEN to use local.

Update: Whoops! I stand corrected (thanks, shotgunefx)! Starting with Perl 5.5, it became possible to localize arbitrary anonymous variables (e.g., local $obj->get_handle()->{some_property}->get_param("param1");). I never actually had a use for (and therefore never tried) localize a lexical; I suppose I just assumed that they could.

Apologies to all for suggesting that localizing a lexical was possible.


If you GIVE a man a fish you feed him for a day
TEACH him to fish and you feed him for a lifetime

In reply to Re(4) Whether to use local() by dmmiller2k
in thread Whether to use local() by jerrygarciuh

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