use v5.12;
use warnings;
use Data::Dump qw/pp dd/;
use Test::More;


sub escape_metas {
    my ( $meta,$e ) = @_ ;

    $e //= '\\';                        # default backslash

    my $ee ="\Q$e";                     # don't mess my regex

    s[ (?|
           $ee($ee)                     # ignore double escapes 
       |
           $ee($meta)                   # keep single escapes
       |
           ($meta)                      # escape meta
       )
    ]
     [$e$1]xgr;
}



my $e = '\\';                           # escape code
my $m = '/';                            # to be escaped



for ("$m", "$e$e$m", "$e$e$e$e$m" ) {
    my $got = escape_metas($m,$e);
    is( $got, "$e$_" , "escaping $_ -> $got");
}

for ("$e$m", "$e$e$e$m" ) {
    my $got = escape_metas($m,$e);
    is( $got, $_ , "ignoring $_ eq $got");
}


done_testing;

##</code><code>##

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/escapism.pl 
ok 1 - escaping / -> \/
ok 2 - escaping \\/ -> \\\/
ok 3 - escaping \\\\/ -> \\\\\/
ok 4 - ignoring \/ eq \/
ok 5 - ignoring \\\/ eq \\\/
1..5