$STRING =~ tr/\x00-\xFF//c   # Can be used as a boolean.

##</code><code>##

sub isASCII { !!( $_[0] =~ tr/\x00-\x7F//c ) }

##</code><code>##

my $ascii = encode( "ASCII", "\N{LATIN SMALL LETTER A}" );
isASCII( $ascii )         # true (OK. true positive)

my $decoded_text = "\N{U+2660}";
isASCII( $decoded_text )  # false (OK. true negative)

my $temperature = 18;
my $data_packet = pack( 's>', $temperature );
isASCII( $data_packet )   # true (Error. false positive)