\N escapes are seen pre-interpolation (update: rather, at the same time as variable interpolation, which means the \N{...} is parsed as an entire entity, and the contents are treated as raw data). I suggest chr(charnames::vianame($name)).

Update: you could also use eval() as shown below, but I think my previous solution is far more appropriate.

sub charname_to_char {
  my $name = shift;
  eval qq{
    # because $^H{charnames} doesn't propogate into here
    use charnames ':full';
    "\\N{$name}"
  };
}
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