use strict;
$MyClass::foo = 'boink';

sub bar {
    no strict; # strict turned off for this sub
}
...  # strict again in effect
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They live in files which are imported, like modules - but they act like switches and turn on and off certain flags at compile time, for their lexical scope.

There's no reason to turn strict or warnings off for a "working" version, once the "development" versions behave with them. Again: *no* *reason*. And in case you ask again - no, there is no reason, period.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

No.

The following works:

my $DEBUG;
BEGIN { $DEBUG = $ARGV[0]; }
use if $DEBUG, 'strict';
# <-- The compiler is here because
#     Perl executes use statements
#     as soon as it compiles them.
#     "strict" will work until the
#     end of this block (namely
#     until the end of the file).
print($test);   # strict error if $DEBUG
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The following works:

my $DEBUG;
BEGIN {
   $DEBUG = $ARGV[0];
   if ($DEBUG) {
      require strict;
      import strict;
   }
}
# <-- The compiler is here because
#     Perl executes BEGIN blocks
#     as soon as it compiles them.
#     "strict" will work until the
#     end of this block (namely
#     until the end of the file).
print($test);   # strict error if $DEBUG
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The point, however, is that the following doesn't work:

my $DEBUG;
BEGIN {
   $DEBUG = $ARGV[0];
   if ($DEBUG) {
      eval <<'__EOI__';
         use strict;
         # <-- The compiler instance used
         #     by eval is here because
         #     Perl executes use statements
         #     as soon as it compiles them.
         #     "strict" will work until the
         #     end of this block (namely
         #     until the end of the code
         #     passed to eval).
__EOI__
   }
}
print($test);   # No strict error
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Actually, I thought the second one wouldn't work either, but it does for the reason I gave in the comments.