Using my module HTTP::Request::FromCurl or it's associated online website, curl2lwp, you can conveniently convert any curl command to Perl.

#!perl
use strict;
use warnings;
use WWW::Mechanize;
use HTTP::Request;


my $ua = WWW::Mechanize->new();
my $r  = HTTP::Request->new(
    'GET' => 'https://api.coindesk.com/v1/bpi/currentprice/usd.json',
    [
        'Accept'     => '*/*',
        'Host'       => 'api.coindesk.com:443',
        'User-Agent' => 'curl/7.55.1',
    ],

);
my $res = $ua->request( $r, );


__END__

Created from curl command line
curl -s https://api.coindesk.com/v1/bpi/currentprice/usd.json
[download]

Reading and extracting the rate from the JSON file is easiest done using the JSON module (or JSON::PP or JSON::XS or ...), which convert the JSON to a nice data structure which you can then traverse from Perl.