Greeting Monks,

I’m experimenting with using the built-in `DB` package to build a lightweight call tracer. I’ve previously done something similar using **Hook::LexWrap** and **Module::Info**, but this time I’d like to catch **entry and exit** events for subroutines directly through Perl’s built-in debugging interface.

Here’s the minimal prototype I’ve tried:

                                                                      
+        
package Devel::Lite;

use strict;
use warnings;
use Time::HiRes ();

our $IN_TRACE = 0;

sub DB::sub {
    return &$DB::sub(@_) if $IN_TRACE;
    local $IN_TRACE = 1;

    my $sub = $DB::sub;
    my @args = @_;

    print STDERR ">> entering $sub\n";
    my @ret;
    eval { @ret = &$sub(@args); };
    print STDERR "<< leaving $sub\n";

    die $@ if $@;
    return wantarray ? @ret : $ret[0];
}
1;

[download]

Then I run it like this:

perl -I. -d:TobyLite -e "sub x { print qq{okay\n}; } x();"

[download]

…but it loops endlessly and never reaches my test sub.

If I insert a print inside the loop, I see repeated `"Time::HiRes::time"`, suggesting recursion into the debugger hook itself.

I’ve tried things like skipping `Devel::Lite` and `Time::HiRes` calls explicitly, and guarding with `$IN_TRACE`, but the loop persists.

What I’d like to understand is:

1. Why is `DB::sub` being triggered recursively despite the guard?

2. What’s the correct minimal pattern for tracing sub entry/exit safely?

3. Bonus: how best to distinguish user code from CPAN/core subs to keep the trace clean?

Environment:

This is perl 5.38.x built for MSWin32-x64-multi-thread

Running on Windows 10.

Any insight or examples would be appreciated — I’m not trying to build a full debugger, just a simple tracer that logs the call tree.

TIA

PS: So far this has stumped (in no order) ChatGPT, claude, and Gemini.

In reply to Trouble writing a simple tracer using the DB package (loops endlessly) by clueless newbie

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