Given the fixed hash %validLanguages (note that there is a level of hell for devising that name),
Start the sub definition and shift in one argument:

sub is_valid_list {
    my $text = shift;
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now split the string on combinations of a character class of comma and whitespace:     my @langs = grep {defined} split /[\s,]+/, $text; look for nonexistent keys and return the negation of scalar of that list - gives zero if some were found, one if none were. End of sub.

    ! grep { ! exists $validLanguages{$_} } @langs;
}
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That's untested but I figure it will work.

Update: The original failed for strings with [\s,]+ at the ends. Inserted grep {defined} to take care of that.

After Compline,
Zaxo