$filename = "~/madam";
$filename =~ s{
    ^ ~     # find a leading tilde
    (       # save this in $1
    [^/]    # a non-slash character
    *       # repeated 0 or more times (0 means me)
    )
  }{
    $1
    ? (getpwnam($1))[7]
    : ( $ENV{HOME} || $ENV{LOGDIR} )
  }ex;

print $filename, "\n";