Hi

I hacked my perldebugger in a way to act as a REPL, i.e. instantly printing the returnvalue from every executed line.¹

from time to time I have weird output, like

  DB<100> sub tst {}
 => 0

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which is wrong, since sub returns nothing (i.e. empty list in list context)

$ perl -e 'use Data::Dumper; print Dumper eval "sub tst {}"'
$

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digging into the code of perl5db.pl helped me identifying the source of the problem:

The debugger automatically prepends code to the command-line before evaling it.

Essentially the directive '$^D=$^D;' causes the problem

$ perl -e 'use Data::Dumper; print Dumper eval "\$^D=\$^D;sub tst {}"'
$VAR1 = '0';

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I'm not sure why this is happening, IMHO it's a bug in the implementation of eval and/or debug mode.

The original line in perl5db.pl is

$evalarg = "\$^D = \$^D | \$DB::db_stop;\n$cmd";

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my workaround now is using an explicit do { ...}

$evalarg = "\$^D = \$^D | \$DB::db_stop;\ndo {$cmd}";

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:~$ perl -e 'use Data::Dumper; print Dumper eval "\$^D=\$^D;do {sub ts
+t {}}"'
:~$

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This should hopefully not cause any new side effects... but is still a weird phenomenon.

Anyone here who can enlighten me, or give me a better workaround?

Cheers Rolf

( addicted to the Perl Programming Language)

¹) see also YAPC talk

In reply to Hacking Debugger bugs connected to $^D by LanX

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