#!/usr/bin/env perl
use strict;
#use warnings;
use 5.020;

sub function {
    say "at function:$ENV{THIS}";
}

sub main {
    say "at main before setting:$ENV{THIS}";
    local $ENV{THIS} = 'THAT';
    say "at main after setting:$ENV{THIS}";
    function();
    say "at main after function:$ENV{THIS}";
    return 0;
}

say "before main:$ENV{THIS}";
main(@ARGV);
say "after main:$ENV{THIS}";

__END__
Output:
$ ./local.pl
before main:
at main before setting:
at main after setting:THAT
at function:THAT
at main after function:THAT
after main: