If your program accepts a regular expression pattern, either from a user input or another module, you need to check that the pattern you receive is valid or not. To check for a valid pattern, apply the pattern against an empty string and wrap the expression in an eval.

sub my_func {
    my ($pattern) = @_;

    eval {
        "" =~ $pattern;
    };
    if ($@) {
        die "Something wrong with your pattern: $pattern";
    }

    # Otherwise, pattern is good and use it here.
}

Here’s how to determine what modules have been installed after the original Perl installation, hence showing those modules not part of the core installation. Type this from a command line:

perldoc perllocal

Often, I need to compare two variables but I don’t know if they are numbers or strings. I need to know the type so I can pick the proper comparison operators, i.e., == or eq, > or gt.
So, here’s how to do it:

~$x ne ~"$x" ? 'numeric' : 'string'

~ is the bitwise negation operator (see perlop). It does not negate integers and strings in the same way.

Examples:

$ perl -e 'print ~1, "n"'
4294967294
$ perl -e 'print ~"1", "n"'
Œ

(character which ASCII code is 255 – ord(‘1’))

To create a directory path programatically:

use File::Path;

mkpath "/usr/local/apache/htdocs/articles/2003";

This will create the 2003 directory and all parent directories as needed. This is the same as mkdir -p command.

Several times I needed a timestamp for whatever reason. Here’s one that will return a scalar containing the current timestamp as in 03Jun19-114251.

sub create_timestamp {
    my ($sec, $min, $hour, $mday, $mon, $year) = localtime;
    my $month = (qw(Jan Feb Mar Apr May Jun
                    Jul Aug Sep Oct Nov Dec))[$mon];
    $year %= 100;

    my $timestamp = sprintf("%02d%s%02d-%02d%02d%02d",
                         $year, $month, $mday, $hour, $min, $sec);

    return $timestamp;
}