use Text::Balanced qw(extract_codeblock);

# get source code in this variable
my $source = ....;

# regexp finds where method is defined and sets "pos" (see perldoc -f pos),
# than extract_codeblock extracts following codeblock
if($source =~ /public \s+ void \s+ readExternal \s+ \( .*? \) \s+ /gx) {
    my $sub_source = extract_codeblock($source);
    print $sub_source;
}