You might be familiar with Bash's set -x. Or even Basic's TRON command, which may have been related to the movie 'Tron'.

(note: Some modules below will only enable tracing within the CURRENT package. To fix this — to enable tracing everywhere:
use everywhere '...';
)

automatically trace things, without having to add code in every subroutine

• Devel::Trace (20 ++s, last updated Mar 2012, first released Sep 1999)
  • good if you just need specific line numbers, but minimal variable contents
  • the output only goes to STDERR, if you need it to go somewhere else, then manually redirect STDERR
• Data::Dump::Trace (41 ++s, last updated Jun 2021, first released 1998)
• Devel::DumpTrace (7 ++s, last updated May 2019, first released 2011)
  • good if you need LOTS of variable contents
• Devel::Trace::More (2 ++s, last updated Aug 2011, first released Oct 2009)
• Devel::Trace::Fork (last updated Sep 2010, first released ??)
• Log::Trace (last updated Nov 2005, first released ??)
• Debug::Trace (5 ++s, last updated Nov 2013, first released Oct 2002)
• Acme::Echo (last updated Apr 2007, first released Aug 2006)
• Devel::STrace — although it's called "strace", it has nothing to do with that system utility, and it only traces Perl subs (last updated Aug 2006, first released ??)
• even more detailed tracing
  • Runops::Trace — trace Perl opcodes (2 ++s, last updated Jan 2011, first released Oct 2005)
• other funky stuff
  • Devel::Spy — logs all accesses to your objects and data (1 ++, last updated Jul 2010, first released ~2006)
  • Tie::Trace — watch specific variables that you 'tie' (8 ++s, last updated Jan 2017, first released Aug 2006)
• interfaces to truss/strace/etc (ie. can be used for non-Perl programs)
  • Sys::Trace (last updated Jul 2010, first released Jun 2010)

trace whenever a variable gets changed

• Data::Trace (2++, last updated Oct 2024)

only displays a full stack (with subroutine arguments) for very specific conditions... not every line

• Carp::Trace
• Carp::REPL

using Devel::DumpTrace in a CGI script

#!/usr/bin/perl -d:DumpTrace
    

BEGIN {
    # if Devel::DumpTrace is loaded, direct output to trace.txt
    if ($INC{'Devel/DumpTrace.pm'} && !$^C) {
        no warnings 'once';         # avoid name "used only once" warnings
        open $Devel::DumpTrace::DUMPTRACE_FH, '>', '/var/tmp/trace.txt';
        $Devel::DumpTrace::EXCLUDE_PKG{'DB_File'} = 1;
    }
}

library-specific logging/tracing

• DBI
  • DBI::Log
  • DBI's own tracing facility
  • other possibilities include [1] [2]
• DBIx::Class
  • DBIx::Class::Storage::Debug::PrettyTrace
  • DBIx::Class::UnicornLogger
• LWP ⇒ LWP::ConsoleLogger