See code below... is the current selected filehandle (usually STDOUT, but not in the magic while(<>)) some perl default variable? I looked in perlvar but didn't see anything. How does this inplace edit magic work? Where is it documented? 

use strict;
use warnings;
use Test::More qw(no_plan);

#create a.txt contining "a"
open F, "> a.txt";
print F "a";
close F;

#fails because no substitution was made
#prints to STDOUT
process_file(); 


local $^I = ".bak";

#now it works.
#prints to "a.txt" as we want. But how does it know to print there?
write_and_read_file(); #ok

sub process_file {
    
    @ARGV = qw(a.txt);
    
    local $/; # slurp it
    #changes a.txt so contains "b"
    #the diamond operator operates on @ARGV by default
    while (<>) {
        #$_ now contains the entire file, alter at will
        s/a/b/g;
        #How does the print operator know what file handle to print to
+? 
        #Was this selected somewhere by magic?
        #Where is this documented / explained?
        print;
    }

    #need to reset @ARGV, I guess it got wiped out somewhere in that w
+hile.    
    @ARGV = qw(a.txt);
    while (<>) {
        is($_,"b");
    }
    
}

[download]
Outputs: 
bnot ok 1
#     Failed test (diamond.pl at line 35)
#          got: 'a'
#     expected: 'b'
Undefined subroutine &main::write_and_read_file called at diamond.pl l
+ine 15, <> line 1.
1..1
# Looks like you failed 1 tests of 1.
# Looks like your test died just after 1.

[download]
(Inspired by Re: Shell trick to edit many files with perl)

In reply to how does print know what filehandle to use, when in a diamond in-place edit loop? by tphyahoo

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