G'day perldigious,

I looked at this from the perspective of using a lexical filehandle (which is good) but then giving that filehandle global scope (which is less good). Consider this version of your code:

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

{
    open(my $first_fh, "<", "file1.txt");
    my @file1_lines = <$first_fh>;
}

{
    open(my $second_fh, "<", "file2.txt");

    while (my $line = <$second_fh>) {
        my @data = my_sub($first_fh, $line);
    }
}

sub my_sub { 1 }
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All the I/O involving $first_fh is performed in the first anonymous block. On exiting that block, $first_fh goes out of scope, Perl automatically closes it for you, and strict will warn you about it being an argument to &my_sub.

Running that code gives a compile-time error:

Global symbol "$first_fh" requires explicit package name ... line 16.
Execution ... aborted due to compilation errors.
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Now's the time for that Oops! moment you were hoping for but didn't get. :-)

After s/first/second/ the code compiles cleanly, as evidenced by the runtime error I get:

Can't open 'file1.txt' for reading: 'No such file or directory' ... li
+ne 8
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As a general rule, use lexical variables in the smallest scope possible.