use DateTime::Format::Strptime;

my $Strp1 = DateTime::Format::Strptime::->new(
    pattern     => '%A %B %d %H:%M:%S %Y',
    locale      => 'en_US',
    on_error    => 'croak',
);

#YYYY_MM_DD_HH_MM_SS 
my $Strp2 = DateTime::Format::Strptime::->new(
    pattern     => '%Y_%m_%d_%H_%M_%S',
    locale      => 'en_US',
    on_error    => 'croak',
);

my $dt = 'Sat Sep 20 17:44:55 2003';
warn $dt,$/;

die $Strp2->format_datetime($Strp1->parse_datetime( $dt )),$/;

__END__
Sat Sep 20 17:44:55 2003
2003_09_20_17_44_55