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Perl Cheat Sheet

Basic Functions, Operators, Regex

Have a look at https://www.pcwdld.com/perl-cheat-sheet

Installing Modules from CPAN

Invoke CPAN shell

perl -MCPAN -e shell

Install a module

cpan[1]> install Some::Package

List CPAN installed modules

$ cpan -l

Catching exceptions

eval {
   // your code which might die()
   1;
} or do {
   warn("Failed: $_");
};

UTF-8 CGI Handling

From Proper UTF-8 CGI handling:

    use utf8;
    use open ( ":encoding(UTF-8)", ":std" );
    use CGI ("-utf8");

Daemonizing in Perl

From Always double fork:

    use POSIX;

    sub daemonize
    {
        fork and exit;
        POSIX::setsid();
        fork and exit;
        umask 0;
        chdir '/';
        close STDIN;
        close STDOUT;
        close STDERR;
    }

JSON+XPath

Ad-hoc JSON filtering with XPath

    lwp-request https://example.com/api-endpoint | perl -MJSON::XS -MData::DPath=dpath -E 'say join(",", dpath("//status")->match(decode_json(join("",<STDIN>))))'

Debugging

Value Debugging in Perl

This is usually done with Data::Dumper

    use Data::Dumper;

    # long form
    print Data::Dumper->Dump([$var], [qw(var)]);   # scalar variable
    print Data::Dumper->Dump([$var]);              # scalar variable without name

    # short form
    print Dumper(\@var);           # array variable
    print Dumper(%var);            # hash as simple list
    print Dumper(\%var);           # hash nicely indented
    print Dumper($var);            # arbitrary reference

Performance Debugging in Perl

nyprof is a good debugger that supports flame graphs and a very useful static HTML client. To create HTML from a trace

nytprofhtml --file trace.out --open trace.out --out result

To run CGIs with ad-hoc request parameters

QUERY_STRING='<some query>' REQUEST_METHOD=GET REMOTE_USER=<user> perl -d:NYTProf <cgi script>

Misc

  • Case-insensitive sorting

     sort { "\L$a" cmp "\L$b" } @mylist

  • Regex - Always use named capture groups and %+, for example:

      if($str =~ /(?<field1>\w+)\s+(?<field2>\w+)(\s+(?<field3>\w+))?/) {
        %result = %+;
  }

  • Perl - Random Values:

      int(rand(10))

  • Perl - Fix for “perl: warning: Setting locale failed.”

      locale-gen en_US.UTF-8   # Insert listed locale(s)