regex - Perl: Why doesn't eval '/(...)/' set $1?

Question

If a regular expression match occurs inside an eval, changes to the capture-related variables ($1, etc.) are not visible in the outside environment. Is this a bug?

perlop and perlre don't seem to mention any such restriction.

For example:

 use strict; use warnings;
 $_ = "hello";
 eval '/(.*)/';
 print "GOT: $1
";

gives:

Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
GOT:

A more succinct demo is:

perl -we '$_="foo"; eval q(/(.*)/;) ; print "GOT:$1
";'

Answer 1

Documentation proof that localized variables are the issue here is in perlvar of 5.14.0:

These variables are read-only and dynamically-scoped, unless we note otherwise.

The dynamic nature of the regular expression variables means that their value is limited to the block that they are in [...]

Note that this bit of documentation is absent from the 5.12.4 perldoc.

---

The problem is localized variables. My copy of perldoc -f eval (5.12.4) has this to say:

The assignment to $@ occurs before restoration of localised
variables, which means a temporary is required if you want to
mask some but not all errors: [...]

The manpage doesn't make an explicit statement for all such special global variables (like $1, $&, and probably others), but block localization and subsequent restoration is what appears to happen here.

The variables are assigned to inside the eval and the original values are restored once the eval block is left.

use strict; use warnings;
use Test::More;
use constant V => 'hello';

$_ = V;

note '*** block eval';
eval {
        is $_, V, 'input ok';
        /(.*)/;
        is $&, V, 'in eval'; is $1, V, 'in eval';
};
is $&, V, 'after eval'; is $1, V, 'after eval';

note '*** without eval';
is $_, V, 'input ok';
/(.*)/;
is $&, V; is $1, V;

done_testing;