A string won't always function as a regexp to find itself. What people mean by using substr instead is to do something like:

sub subpos { # find the offset of any substring 
    my ( $checkstring, $substring ) = @_;
    my $length = length( $substring );
    my $limit = length( $checkstring ) - $length; 
    for ( my $i = 0; $i < $limit; $i++ ) {
        ( substr( $checkstring, $i, $length ) eq $substring )
          and return $i;
    }
    undef(); # or alternatively return -1 for not found
}
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length.