Actually, I think the better way would be to return true on the first non-ASCII character, and treat "true" as the condition to be avoided:

sub nonascii
{
    local $_ = shift;
    /[^\0-\x7f]/;   # or /[^[:ascii:]]/
}

for my $test ( split( " ", "foo bar b\xe1 baz" )) {
    next if ( nonascii( $test ));
    print "$test\n";
}
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(update: this is just a stylistic difference relative to  $str !~ /[^\0-\x7f]/ -- to avoid the cognitive challenge of the double-negative)