That is hard to improve on. You may like to give it a meaningful name and precompile it:

my $ungood = qr/[^a-zA-Z1-9 ]/;
warn "Bad Text!" if $string =~ $ungood;
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If you want to count tabs and newlines as spaces, you can use the \s character class instead of your space, and add the s qualifier to the regex.

It could be worthwhile someday to recast your character class as unicode, something like $ungood = qr/[\P(IsAlnum)\P(IsSpace)]/s; $ungood = qr/[^\p(IsAlnum)\p(IsSpace)]/s; (Oops!).

After Compline,
Zaxo