pre-commit hook for adding a timestamp to the frontmatter of markdown files

pre-commit

#!/usr/bin/perl
use POSIX qw(strftime);

# this is a pre-commit hook for setting a timestamp in the frontmatter
# of markdown files

my $date = strftime "%Y-%m-%dT%H:%M:%SZ", gmtime;
my @files = `git diff-index --cached --name-only HEAD`;
foreach (@files) {
    if (/\.md$/) {  # maybe HTML, too?
	open FILE, $_ or die $!;
	my @lines = <FILE>;
	close FILE or die $!;
	my $file = join('', @lines);
	if ($file =~ m/---\n(.*)---\n(.*)/s) {
	    my $frontmatter = $1;
	    my $body = $2;
	    if (!($frontmatter =~ s/^date: .*$/date: $date/m)) {
		$frontmatter = $frontmatter . "date: $date\n" 
	    }
	    open FILE, ">$_" or die $!; 
	    print FILE "---\n$frontmatter---\n$body";
	    close FILE or die $!;
            system "git add $_";
	}
    }
}