That's not a warning, but a compilation error. (Warnings print out a complaint, but your script keeps running. Errors stop your script in its tracks.) However, I don't get that error running the code you posted.

This error occurs if you use strict (in particular the vars component of strict) and attempt to use a variable which is not either (a) declared, or (b) includes a package name.

$ perl -Mstrict -E'$hello = "Hi"; say $hello'
Global symbol "$hello" requires explicit package name at -e line 1.
Global symbol "$hello" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
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Solution 1: declare your variable with my or our. (Check the fine documentation for the difference between them.)

$ perl -Mstrict -E'my $hello = "Hi"; say $hello'
Hi
$ perl -Mstrict -E'our $hello = "Hi"; say $hello'
Hi
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Solution 2: include a package name in the variable's name.

$ perl -Mstrict -E'$World::hello = "Hi"; say $World::hello'
Hi
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Solution 3: don't use strict. (Using strict is generally a good idea, but sometimes you really do know what you're doing, and just want it to stop bothering you.)

$ perl -Mstrict -E'no strict qw(vars); $hello = "Hi"; say $hello'
Hi
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