my $filename = "~user2/tmp";

if( $filename =~ m@^\~([^/])+@){
     my $home = (getpwnam($1))[7];
     if( $home){
          $filename =~ s@^\~[^/]+@$home@;
     }
}else{
     $filename =~ s@^\~@$ENV{'HOME'}@;
}