Given many, many files named like this: my_file_1.php, how can I rename them to be named like this: my-file-1.php 

use strict;
use warnings;
use 5.010;

my $dir = shift or die "Usage: $0 DIR";

use Path::Iterator::Rule;
use Path::Tiny qw(path);
my $rule = Path::Iterator::Rule->new;

for my $file ( $rule->all( $dir ) ) {
    #say $file;
    my $pt = path $file;
    #say $pt->basename;
    if ($pt->basename =~ /\.php$/) {
        my $newname = $pt->basename;
        $newname =~ s/_/-/g;
        #say "rename " . $pt->path . " to " . $pt->parent->child($newname);
        rename $pt->path, $pt->parent->child($newname);
    }
}

Save this as rename.pl and then run it on the command line perl rename.pl path/to/dir.

Alternatively, save this as rename.pl, replace the my $dir ... line by my $dir = "/full/path/to/dir"; and then you can run it without passing the directory name on the command line.

On Windows the path name should be either using slashes: my $dir = "c:/full/path/to/dir";, or using pairs of back-slashes: my $dir = "c:\\full\\path\\to\\dir";.

Path::Iterator::Rule will allow us to traverse the directory tree, so we can rename files in the whole tree.

Path::Tiny helps us extracting the directory name and building the new name.

Some commented out print-statement were left in, to make it easier to follow what's happening.