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Package names are sometimes an exception to this rule. Perl informally
+ reserves lowercase module names for ``pragma'' modules like integer 
+and strict. Other modules should begin with a capital letter and use 
+mixed case, but probably without underscores due to limitations in pr
+imitive file systems' representations of module names as files that m
+ust fit into a few sparse bytes.

Undefined subroutine &pm::security::banned called at pm/user.pm line 1
+36

Do not export anything else by default without a good reason!

Exports pollute the namespace of the module user. If you must export t
+ry to use @EXPORT_OK in preference to @EXPORT and avoid short or comm
+on symbol names to reduce the risk of name clashes.

If it returns a true value, then no objects of that class will be clon
+ed; or rather, they will be copied as unblessed, undef values.

package Pm::Security;

...
}

1;

#!usr/bin/perl
use say;
...

$ perl main.pl
Second~First

$ perl main.pl
Can't locate Pm/Security.pm in @INC (you may need to install the Pm::S
+ecurity module) (@INC contains: /home/tinyos/perl5/lib/perl5/5.24.1/x
+86_64-linux-gnu-thread-multi /home/tinyos/perl5/lib/perl5/5.24.1 /hom
+e/tinyos/perl5/lib/perl5/x86_64-linux-gnu-thread-multi /home/tinyos/p
+erl5/lib/perl5 /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.24.1 
+/usr/local/share/perl/5.24.1 /usr/lib/x86_64-linux-gnu/perl5/5.24 /us
+r/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.24 /usr/share/perl/5.2
+4 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base) at ma
+in.pl line 7.
BEGIN failed--compilation aborted at main.pl line 7.

LinuxThreads is now obsolete on Linux, and caching getpid() like this 
+made embedding perl unnecessarily complex (since you'd have to manual
+ly update the value of $$), so now $$ and getppid() will always retur
+n the same values as the underlying C library.