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

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

my $ascii = encode( "ASCII", "\N{LATIN SMALL LETTER A}" );
isASCII( $ascii )         # true (OK. true positive)
...
my $temperature = 18;
my $data_packet = pack( 's>', $temperature );
isASCII( $data_packet )   # true (Error. false positive)