I am seeking to expand my understanding of the interplay between BEGIN and END blocks, strictures and scoping.

Assume that I have on my system a file C:/shb.

Let's call this Case 1:

# perl
BEGIN {
    $shbstatus = (-f "C:/shb") ? 1 : 0;
}
END {
    $shbstatus  ? print "shb identified\n"
                : print "shb was not identified\n";
}
use warnings;

print "XYZ\n";
print "$shbstatus\n";
[download]

I expect the output to be ...

XYZ
1
shb existed
[download]

... and indeed it is.

Case 2: I modify the variable declared inside the BEGIN block to make it private.

# perl
BEGIN {
    my $shbstatus = (-f "C:/shb") ? 1 : 0;
}
[download]

Since $shbstatus is now private to the BEGIN block, I expect the output to be ...

XYZ
Use of uninitialized value in concatenation (.) or string at begin.pl 
+line 15.

shb was not identified
[download]

... and so it is. C:/shb still exists; it just wasn't identified.

Case 3: I restore the variable declared inside the BEGIN block to global status, but apply strictures before the BEGIN block.

# perl
use strict;
BEGIN {
    $shbstatus = (-f "C:/shb") ? 1 : 0;
}
[download]

Part of me says, "$shbstatus is inside a BEGIN block, which gets executed first, so it won't throw an error." Another part of me says, "No, use strict precedes the BEGIN block in the file, so $shbstatusmust be qualified." It turns out that the latter is correct, as the output is:

Global symbol "$shbstatus" requires explicit package name at begin.pl 
+line 4.
BEGIN not safe after errors--compilation aborted at begin.pl line 5.
shell returned 9
[download]

Case 4: But now I put use strict inside its own BEGIN block which precedes that containing $shbstatus.

Update: I should have included this for clarity:

# perl
BEGIN {
    use strict;
}
BEGIN {
    $shbstatus = (-f "C:/shb") ? 1 : 0;
}
[download]

Frankly, I wouldn't have expected the output in this case to be any different from Case 3. To my surprise, everything compiles and prints nicely:

XYZ
1
shb identified
[download]

... which implies that the global variable $shbstatus was not restricted by the use strict call inside the first BEGIN block.

Why not?

(I should add that my goal here is to write a test file that first tests for the existence of a file on disk, then does one thing if the file is present (temporarily rename it) and a different thing if it is not (create it), then makes sure that the presence or absence of the file on the disk is unchanged at the end of the test notwithstanding any dies or croaks in the interim. I've read perlmod and perlsub but am still not entirely clear.)

Thanks in advance.

Jim Keenan

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