Often better to do such things in a single pass.

s{[\\@]([\\@]|[_A-Za-z]+)}{
    my $term = $1;
    if( $term =~ /[\\@]/ ) {
        $term    # Replace repeated escapes with second of pair
    } else {
        print "Extracted code '$term'\n";
        ''       # Replace extracted codes with nothing.
    }
}ge;
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