You need to set your Net::FTP in passive mode when tunneling through a firewall.

my $ftp = Net::FTP->new(
    $arg{host},
    Firewall => 'firewall_host:port_no',
    FirewallType => 4,
    Passive => 1 );
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Then for firewall type 4, do the sequence as said in the Net::Config manual.

USER fwuser
PASS fwpass
OPEN remote.site
USER user
PASS pass
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