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for (split(/:/, $$pattern)) {
    $_ = 'S0' . $_ unless /^S/;
...
    }
    print STDERR "\n";
}

pattern: S51F11S0
0 -> [] 1 -> [51] 2 -> [] 3 -> [11] 4 -> [] 5 -> [0]

pattern: S51F11S0
0 -> [51] 1 -> [11] 2 -> [0]

tr/ //d;
$camel .= $_;