template.pl

#!/usr/bin/env perl

# for formality's sake
use strict;
use warnings;

# Handling the options
use Getopt::Long;

# Default value
my $header    = "src/header";      # default value for header file
my $procedure = "src/procedure";
my $footer    = "src/footer";      # default value for footer file
my $help;

my $result = GetOptions(
    "header=s"         => \$header,
    "procedure|proc=s" => \$procedure,
    "footer=s"         => \$footer,
    "help|h"           => \$help,
);

# show usage if needed and exit 0:
if ($help) {
    &usage;
    exit 0;
}

# check whether the files (header, procedure, footer) exist
if ( not( -e $header ) ) {
    warn "$header does not exist.\n";
    &usage;
    exit 1;
}
elsif ( not( -e $procedure ) ) {
    warn "$procedure does not exist.\n";
    &usage;
    exit 1;
}
elsif ( not( -e $footer ) ) {
    warn "$footer does not exist.\n";
    &usage;
    exit 1;
}

&readFileDoSomething( $header, sub { my $str = shift; return $str } );
&readFileDoSomething( $procedure,
    sub { my $str = shift; return "[memo] " . $str; } );
&readFileDoSomething($footer, sub {my $str=shift; return $str;});

# ------------------
# --- subroutine ---
# ------------------

# readFileDoSomething:
# - explanation: Read from a file line by line,
#                modify it according to the callback function,
#                and print these lines.
# - arguments: filename and callback in this order
#   - filename: a string to specify the filename to be read
#   - callback: a reference to a subroutine.
#               The subroutine is called with each line of the filename given.
#               The subroutine must be return string.

sub readFileDoSomething {
    # arguments
    my ( $file, $callback ) = @_;

    # read file
    open( my $fh, "<", $file )
      or die "Cannot open $file: $!";

    # printing the contents of the file
    while ( my $line = readline $fh ) {
        print &$callback($line);
    }
}

sub usage {
    print "Usage: blahblahblah\n";
}