In effect some special variable can be aliased and still works properly ( as $" for example ) and others cannot ( as $\ and $1 ):

use strict;
use warnings;

print $/.'aliasing output record separator.  $\\'."\n";
for $\ (qw( a b )) {
    # this does not works
    print "\$\\ is ->$\<- but is not appended\n";
}


print $/.'aliasing output list separator $"'."\n";
for $" (qw( a b )) {
    # $" is aliased to a then b,c..
    # BUT is still the output list separator
    print "@{[1, 2, 3]}\n";
}

print $/.'aliasing eval error $@'."\n";
for $@ (qw( a b )) {
    print "now \$@ is: $@\n";
    # no need to localize $@
    eval '2 / 0';
    print "then \$@ is: $@";
}

print $/.'aliasing  first captured match $1'."\n";
for $1 (qw( a b )) {
    
        print "now  \$1 is: $1\n";
        
        'XXX' =~ /(.)/;
        print "then \$1 is WRONG after a match: $1\n";
        
        {
            local $1;
            if ('YYYY' =~ /(.)/){ 
                print "then \$1 is wrong even if \$1 is localized: ".(
+$1 ? $1 : 'UNDEF')."\n";
            }  
                
        }
}

my $text =<<EOT;
first
second
SEP
fourth
SEP
last
EOT

print $/.'input record separator $/'."\n";
for $/ (qw( \n SEP )) {
    print "now \$/ is: $/\n";
    print "LINE:->$_<-\n" for split $/,$text;
}

#OUTPUT
aliasing output record separator.  $\
$\ is ->a<- but is not appended
$\ is ->b<- but is not appended

aliasing output list separator $"
1a2a3
1b2b3

aliasing eval error $@
now $@ is: a
then $@ is: Illegal division by zero at (eval 1) line 1.
now $@ is: b
then $@ is: Illegal division by zero at (eval 2) line 1.

aliasing  first captured match $1
now  $1 is: a
then $1 is WRONG after a match: a
then $1 is wrong even if $1 is localized: UNDEF
now  $1 is: b
then $1 is WRONG after a match: b
then $1 is wrong even if $1 is localized: UNDEF

input record separator $/
now $/ is: \n
LINE:->first<-
LINE:->second<-
LINE:->SEP<-
LINE:->fourth<-
LINE:->SEP<-
LINE:->last<-
now $/ is: SEP
LINE:->first
second
<-
LINE:->
fourth
<-
LINE:->
last
<-
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L*

PS anyway we all know is a stupid thing to do ;)