This works for the sample data you provided.

open (FILE, "sample.txt") || die "cannot open sample.txt";
while (<FILE>) {
s/<\?--1-Stock3of1-->//;
s/<b>This is option Stock 3 of 1<\/b>//;
s/<\/\?--1-Stock3of1-->//;
push (@words, $_);
}
close (FILE) || die "cannot close sample.txt";

open (FILE, ">sample.txt") || die "cannot open sample.txt";
foreach (@words) {
print FILE;
}
close (FILE) || die "cannot close sample.txt";
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