This may work (UNTESTED):

#!/usr/bin/perl
use strict;
use warnings;

use File::Find;

@ARGV == 3 or die "usage: $0 DIR OLD_EXT NEW_EXT\n";

my $dir = $ARGV[ 0 ];
my $old = $ARGV[ 1 ];
my $new = $ARGV[ 2 ];

my @list;
find sub {
    push @list, $File::Find::name if -f && /\.\Q$old\E\z/;
    }, $dir;

foreach my $old_name ( @list ) {
    my $new_name = $old_name =~ s/\.\Q$old\E\z/.$new/r;
    rename $old_name, $new_name or die "Cannot rename '$old_name' beca
+use: $!";
    }
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