while ( <$fh> ) {
    chomp;           # Remove trailing newlines (a subset of \s)
    s/\s//g;         # Remove ALL whitespace.
    next if /^#/;    # Skip comments.
    next if /^\s*$/; # Skip lines with space, or lines with nothing.
                     # This doesn't make sense because you
                     # already removed all space.  But it
                     # works because you allow it to match
                     # 'nothing' too.
    next if $_ eq "";# This line is never true, because
                     # you already skipped lines with nothing
                     # in the previous statement.
#......
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In other words, much of what you're doing there is either redundant or otherwise peculiar.

Next, your "read_cal" function is being called with two arguments, but within the function, you're only using the first argument.

I didn't even look at what you're doing in the if (!$mode... code, because I don't see $mode being set.

Also, you're only printing out hash elements with a value equal to one, though it might be simpler to just check for the existance of a hash element.

Update: None of these are the central problem. Congrats Roger for finding the straw that breaks this camel's back. I found a lot of other issues, but you found the big one. I'd recommend considering reworking the code a bit based on the suggestions people have given in this thread though, as the result will be cleaner.