A couple of remarks here.

• If 'use strict' makes your program break, then take the time to fix what strict is complaining about. It will never complain about something that it shouldn't. (At least, not at the level you say you're programming at.)
• Now that you have posted even just an example piece of code, we now know exactly what it is you're having a problem with. There are a number of potential problems you might have been having. Now that you have an example, I can give you a suggestion.

Try the following code. Run it "as is" once, to understand what it's doing. (You might have to change where the Perl interpreter is.)

#!/usr/local/bin/perl

use strict;
use warnings;

my $file_scoped_variable = '';

sub modify_var {
    $file_scoped_variable = "Var Set";
}

sub print_var {
    print "Hello ... " , $file_scoped_variable , "\n";
}

&modify_var;
&print_var;

__END__
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If, when you try that small script, it doesn't work, then tell us why it doesn't work. Don't just say "Sorry, it's broken."