Plan ahead I intend to add some debugging code to HTML::TableExtract::TableState::_taste_text and override HTML::TableExtract's exception handler (I discovered that it traps any die() calls somewhere and doesn't report them). All I want to do here is eval the guts of the subroutine as a block and check $@ afterward. I'm also adding some print() calls into the routine so I can debug some conditional code I've got questions about.

Deparse the method This is a simple oneline routine that deparses a single subroutine.

perl -MB::Deparse \
     -MHTML::TableExtract \
     -le 'print B::Deparse->new->coderef2text(\&HTML::TableExtract::Ta
+bleState::_taste_text)'

{
    package HTML::TableExtract::TableState;
    my($self, $text) = @_;
    my $sc = $self->_skew;
    if ($self->_terminus_trigger and $self->_column_wanted or $$self{'
+umbrella'}) {
        if (defined $text) {
            print STDERR "Add text '$text'\n" if $$self{'debug'} > 3;
            $self->_add_text($text, $sc);
        }
    }
    if (defined $text and $self->_any_headers and not $self->_any_htri
+gger) {
        $self->_htxt($text);
    }
    1;
}
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Alter the source Assume for the moment that I've just captured the method to a variable $source_code. I'd like to wrap the entire method in an eval block like eval { ... }; print STDERR $@ if $@;die $@;. This is a simple text manipulation: $source_code = "eval { $source_code }; print STDERR $@ if $@; die $@;";. Now to add my debugging code after the assignment to $self.

$source_code =~ s/(\$self.*)$/$1

    print "skew: ".\$self->_skew."\\n"
        . "_terminus_trigger: ".\$self->_terminus_trigger."\\n"
        . "_column_wanted: ".\$self->_column_wanted."\\n"
        . "umbrella: \$\$self{'umbrella'}\\n"
        . "text: q[\$text]\\n";\n/ or die "Source patch failed";
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Install the new codeThat'll add in some print statements to the next line after $self is first mentioned. From here the source is read to generate a new subroutine which is copied into the symbol table in the right place. My installation code is careful to check that eval worked because otherwise there's a bug in the source-altering section.

*HTML::TableExtract::TableState::_taste_text =
    eval "sub { package Package::Name; $source_code };" or die "Eval o
+f \$source_code failed: $@";
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From here I just use the program like I normally do except that now I get some diagnostic output back and at a minimum of effort. Here's the complete patching code.

use B::Deparse ();

{   # Fetch the original source code
    my $source_code = B::Deparse->new->coderef2text( \&HTML::TableExtr
+act::TableState::_taste_text );

    # Alter the source to include error trapping and the diagnostics
    $source_code = 'eval { ' . $source_code . ' }; print STDERR $@ if 
+$@; die $@;';
    $source_code =~ s/(\$self.*)$/$1

    print "skew: ".\$self->_skew."\\n"
        . "_terminus_trigger: ".\$self->_terminus_trigger."\\n"
        . "_column_wanted: ".\$self->_column_wanted."\\n"
        . "umbrella: \$\$self{'umbrella'}\\n"
        . "text: q[\$text]\\n";\n/ or die "Source patch failed";

    # (updated - added a package declaration per 'theorbtwo')
    # Create and assign the new method into place.
    *HTML::TableExtract::TableState::_taste_test =
        eval "sub { package HTML::TableExtract::TableState; $source_co
+de; };"
            or die "Eval of \$source_code failed: $@\n$source_code";
}
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_{Just as an aside, Louis_Wu and PodMaster suggested some alternate titles: "Deparse+eval: poking in the dark and hoping it all works out", "What's with all this B::Deparse stuff?" and "I want to be Damian Jr.".}