Question

I want to detect when a user is missing a required module and print a friendly error message explaining what they need to install.

So far I tried putting this at the beginning of my script:

eval {
    use IO::Uncompress::Gunzip qw(gunzip $GunzipError) ;
};
if ($@) {
    die "Error: IO::Uncompress::Gunzip not installed: $@";
}

But Perl seems to die on the “use” line instead of the “die” line and never prints my error message.

Answer 1

What’s happening here is that the module is being used at compile-time, regardless of the fact that it is inside the eval block.

This is also why naab’s suggestion to change from eval BLOCK form to eval EXPR form works as well; the expression is evaluated at run-time.
Changing the use to require will attempt to load the module at run-time:

eval {
    require IO::Uncompress::Gunzip;
    IO::Uncompress::Gunzip->import( qw/gunzip $GunzipError/ ) ;
};
if ($@) {
    die "Error: IO::Uncompress::Gunzip not installed: $@";
}

Output

Error: IO::Uncompress::Gunzip not installed: Can’t locate IO/Uncompress/Gunzip.pm in @INC (@INC contains: C:/Perl/site/lib C:/Perl/lib .) at – line 2.