Perl: what's the meaning of .=~

Question

In a perl script, I occasionally wrote

my $s = "text";
$s .=~ " another text";
print "$s\n";

The expected result text another text was not printed, instead weird text as textߞ ߋ was shown in my terminal.

No doubt: the error was the operator .=~ while indeed, I wanted to write .=

But I'm curious: Why isn't .=~ a syntax error? What's the meaning of this operation?

Answer 1

.=~ are two operators, .= and ~ . The tilde is the binary negation operator, see perlop .

Verification:

$ perl -MO=Deparse,-p -e '$x .=~ $y'
($x .= (~$y));
-e syntax OK

Answer 2

When Perl is doing something you don't understand syntactically, you either B::Deparse or B::Concise to figure it out.

B::Deparse

Running

$ perl -MO=Deparse

on that code yields:

my $s = 'text';
$s .= "\337\236\221\220\213\227\232\215\337\213\232\207\213";
print "$s\n";

B::Concise

Running

$ perl -MO=Concise,-exec 

on that code yields:

1  <0> enter 
2  <;> nextstate(main 1 -:1) v:{
3  <$> const[PV "text"] s
4  <0> padsv[$s:1,2] sRM*/LVINTRO
5  <2> sassign vKS/2
6  <;> nextstate(main 2 -:2) v:{
7  <0> padsv[$s:1,2] sRM
8  <$> const[PV "\337\236\221\220\213\227\232\215\337\213\232\207\213"] s
9  <2> concat[t3] vKS/2
a  <;> nextstate(main 2 -:3) v:{
b  <0> pushmark s
c  <0> padsv[$s:1,2] s
d  <$> const[PV "\n"] s
e  <2> concat[t4] sK/2
f  <@> print vK
g  <@> leave[1 ref] vKP/REFC

In both cases, the answer is the same. You have a literal full of a bunch of weirdo characters. This is the result of compiler applying the unary bitwise negation ~ to the literal at compile time, and storing the result in the parse tree.

Answer 3

When choroba isn't around ;) you can use B::Deparse and ppi_dumper to tell you what you're dealing with ( .= and ~ )

$ perl -MO=Deparse,-p -e " $foo .=~ /bar/; "
($foo .= (~/bar/));
-e syntax OK


$ ppi_dumper foo.pl
PPI::Document
  PPI::Statement
    PPI::Token::Symbol          '$foo'
    PPI::Token::Whitespace      ' '
    PPI::Token::Operator        '.='
    PPI::Token::Operator        '~'
    PPI::Token::Whitespace      ' '
    PPI::Token::Regexp::Match   '/bar/'
    PPI::Token::Structure       ';'