$DEBUG=1;
$DEBUG && warn "Doing foo!\n";
foo();
$DEBUG && warn "Done foo!\n";
#etc

##</code><code>##

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

Devel::Trace::trace('on');  # Enable

print "Hello World\n";

foo();

Devel::Trace::trace('off'); # Disable

foo();

sub foo{ print "Foo\n" }

##</code><code>##

C:\>test.pl 2>out
Hello World
Foo
Foo

##</code><code>##

C:\>more out
>> C:\test.pl:3: Devel::Trace::trace('on');  # Enable
>> D:/Perl/site/lib/Devel/Trace.pm:31:   my $arg = shift;
>> D:/Perl/site/lib/Devel/Trace.pm:32:   $arg = $tracearg{$arg} while exists $tracearg{$arg};
>> D:/Perl/site/lib/Devel/Trace.pm:33:   $TRACE = $arg;
>> C:\test.pl:5: print "Hello World\n";
>> C:\test.pl:7: foo();
>> C:\test.pl:13: sub foo{ print "Foo\n" }
>> C:\test.pl:9: Devel::Trace::trace('off'); # Disable
>> D:/Perl/site/lib/Devel/Trace.pm:31:   my $arg = shift;
>> D:/Perl/site/lib/Devel/Trace.pm:32:   $arg = $tracearg{$arg} while exists $tracearg{$arg};
>> D:/Perl/site/lib/Devel/Trace.pm:33:   $TRACE = $arg;

C:\>