use IO::File;
...

my %fd;
my %files = (
   'somestring' => 'somefilename',
   'someother'  => 'someother',
);

my @items = (
   'somestring.this.content.will.go.into.somefilename',
   'someother.this.will.go.into.someother',
   'notthere.this.will.generate.a.warning',
);

foreach @items {
   my @array = split(/\./);
   if (exists($files{$array[0]})) {
      $fd{$array[0]} ||= new IO::File "> $files{$array[0]}"
         or die "Couldn't open $files{$array[0]}: $!";
      $fd{$array[0]}->print;
   } else {
      warn "No filename defined for $array[0]";
   }
}

$fd{$_}->close for keys %fd;