use POSIX qw(strftime);

... 

my $modtime = (stat($file))[9];
$modtime = $modtime - 86400;
if ( $modtime == "" )
{
	$modtime = "null";
}
else
{
	$modtime = POSIX::strftime ("%B %d",$modtime,0,0,0,0,0,0,0,0);
}

...