perl -Mblib t/quoting.t
[download]

I put in some prints and found where it stops. I put a print right before and after this line and it prints the one before but not the one after: my @shells = grep try_shell($_), qw(sh csh bash tcsh ksh dash ash pdksh mksh zsh fish);

Here I put some prints inside the 'try_shell' sub:

sub try_shell {
    my $shell = shift;
print "line 1\n";
    my $out = eval { capture($shell, '-c', 'echo good') };
print "line 2\n";
    if ($out and $out =~ /^good$/) {
print "line 3\n";
        if ($shell =~ /ksh/) {
print "line 4\n";
            if (defined (my $version = eval { `$shell --version 2>&1` 
+})) {
print "line 5\n";
                if ($version =~ /version\s+sh\s+\(AT\&T\s+Research\)/)
+ {
                    diag "skipping tests for broken AT&T ksh shell!";
                    return undef;
                }
            }
        }
print "line 6\n";
        if ($shell eq '!!fish') {
            diag "TODO: add support for fish shell!";
            return undef;
        }
        return 1;
    }
    return undef;
}
[download]

This is the result:

-bash-3.2$ sudo perl -Mblib t/quoting.t
line 1
line 2
line 3
line 6
line 1
line 2
line 3
line 6
line 1
line 2
line 3
line 6
line 1
line 2
line 3
line 6
line 1
line 2
line 3
line 4
^C
[download]

I think it makes it through the first 4 shells then gets stuck on the 5th. I think ksh is the 5th one based on the line that calls the sub:

my @shells = grep try_shell($_), qw(sh csh bash tcsh ksh dash ash pdks
+h mksh zsh fish);
[download]

Hmm, it is estrange because when you tested the shells by hand no one did hang...

Could you continue with your instrumentation inside the function try_shell? You will find it around the middle of the file.