if (!$line || $!) {
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There's no guarantee that $! is unset at the moment you test it. It might be propagated from elsewhere and might not mean anything here. See perlvar:

$!

If used numerically, yields the current value of the C "errno" variable, or in other words, if a system or library call fails, it sets this variable. This means that the value of $! is meaningful only immediately after a failure:

if (open(FH, $filename)) {
    # Here $! is meaningless.
    ...
} else {
    # ONLY here is $! meaningful.
    ...
    # Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# here $! is meaningless.
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In the above meaningless stands for anything: zero, non-zero, "undef". A successful system or library call does not set the variable to zero.

(emphasis mine)

So you should test use $! immediately after a system call which failed i.e. didn't give the expected results, in your case, if open doesn't return a true value:

my $isOpen = open($handle, $templateLocation) or die "foo: $!";
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Your readline might succeed, but that doesn't reset $! if it was set prior to the successful readline() call.