Though I'd agree that index could be used, testing index's return value for truth is only useful in some circumstances. If $i isn't contained in $func_def, index returns -1, which is a true value, so your code will think it's been found. Similarly, if $i is at the start of the string (as the first example in yours is), index returns 0 (its position in the string), which is a false value.

In order to test if one string contains the other, you need to check that index returns >= 0.

Amended Code:

my $func_def = "char *strcmp(const char *s1, const char *s2)";
foreach my $i ('char *','const char.*','*')
{
  if ( index( $func_def, $i ) >= 0 )
  {
    print "'$func_def' contains '$i'\n";
  }
}
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