PPI is state of the art though I have trouble motivating me to dive into the details...

But if you want to enforce coding styles anyway, you should consider designing appropriate Perl::Critic rules.

update

And of course B::Xref or tools like ctags could be of help.

Here's a quick PPI example:

use v5.12;
use PPI;

my $perl = PPI::Document::->new(do { local $/ = <DATA>; \$/ });

say $_ for @{ $perl->find('PPI::Token::Symbol') };

__DATA__
# Some sample code to analyse

my $foo = 1;
my @bar = (2, 3);
say $bar[0];
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The output includes a variable called $bar which doesn't really exist :-( But that shouldn't be a problem for the OP's use case.

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

Specifically look at
Perl::Critic::Policy::Variables::ProhibitReusedNames - Do not reuse a variable name in a lexical scope
Perl::Critic::Policy::Variables::ProhibitUnusedVariables - Don't ask for storage you don't need.

And Perl::Critic::Policy is xpaths

Something simple like this could be a start (with filenames given on the command line):

use strict;
use warnings;
my %identifiers;
undef $/;
undef @identifiers{ /((?:sub\s+|[\$@%])[a-zA-Z_][a-zA-Z_0-9]*)/g } whi
+le <>;
print join "\n", keys %identifiers;
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but there are more things to find, like module names etc.

It's not clear from the perldoc on the 5.12.2 perl I have handy, but it does support some kind of plugins.