Its documented in perlrun under -i[extension]. Here is an example (based on this)

#!/usr/bin/perl --

use strict;
use warnings;

use open qw' :std :encoding(UTF-8)';

Main( @ARGV );
exit( 0 );

sub Main
{
    local ( *ARGV,  $^I );
    @ARGV = @_;
    $^I = '.orig';
    while(<>){
#~ U+2018 (8216), U+2019 (8217)     &lsquo; &rsquo;     Single quotes 
+(left and right)
#~ U+201C (8220), U+201D (8221)     &ldquo; &rdquo;     Double quotes 
+(left and right)
#~ U+201B (HTML: &#8219;) – single high-reversed-9, or single reversed
+ comma, quotation mark (This is sometimes used to show dropped charac
+ters at the end of words, such as goin?  instead of using goin‘, goin
+’, goin`, or goin'[citation needed])
#~ U+201F (HTML: &#8223;) – double high-reversed-9, or double reversed
+ comma, quotation mark 
    
        y/\x{2018}\x{2019}\x{201B}\x{201C}\x{201D}\x{201F}/'''"""/;
        print;
    }
}
__END__
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