When I do rapid prototyping, I often want the debugger to stop whenever a warning is issued from the Perl interpreter (for example about an undefined scalar). The warning contains a line number which is fine, but when executed in a loop, this is not enough. I need the current context. Only then I can examine the state of the program in the context that produced the problem.

So what to do? In the program I replace the signal handler for SIGWARN with my own handler that checks for the typical format of a Perl warning. I do that because I am interested only in warnings from the Perl interpreter. If this format has been detected, the code branches into a special path where I can set a breakpoint. After that the warning message is printed as before the modification.

Update: thanks to perrin the breakpoint setting is not necessary anymore. Also the BEGIN block can be shifted in the .perldb initialization file (see Re^3: Debugging a program), so the original file remains unchanged.

Example:

use strict;
use warnings;

BEGIN 
{
    $SIG{__WARN__} = sub {
        my $warning = shift;
        if ( $warning =~ m{\s at \s \S+ \s line \s \d+ \. $}xms ) {
            $DB::single = 1;    # debugger stops here automatically
        }
        warn $warning;
    };
    return;
}

my $undefined_scalar;

print "Scalar has this value: $undefined_scalar, \n";
print "end\n";
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