You need to escape the square brackets since you want to match a literal [4], so instead of

    if($line =~ /Handling NSPAdvice for mechanism [4]/) {
    # the RE above would match "Handling NSPAdvice for mechanism 4"
[download]

try this:

    if($line =~ /Handling NSPAdvice for mechanism \[4\]/) {
    # alternative: if ( index($line,'Handling NSPAdvice for mechanism 
+[4]')>=0 ) {
[download]

Analogue:

            if ($line =~ /Authentication mechanism returned [[](\S*)[]
+]/) {
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...to...

            if ($line =~ /Authentication mechanism returned \[(\-?\d+)
+\]/) {
            # the RE above would match e.g. [4] -> $1=4,  [-10] -> $1=
+-10, etc.
[download]

HTH