Why did you use }{ between $count++ and die?

The -n switch to perl wraps the entire script in a loop, as follows:

LINE:
while (<>) {
  ...             # your program goes here
}
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So the provided code deparses to this:

LINE: while (defined($_ = <ARGV>)) {
    $first ||= $_;
    $last = $_;
    ++$count;
}
{
    die "ERROR found $count lines. Need >5" unless $count > 5;
    print "FIRST:$first\nLAST:$last\n";
}
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Note how the "butterfly operator" (that's }{) is used to add some extra bit of code that'll be run after the loop that -n adds finishes.

When i tried to run this code, I got following error message: syntax error at -e line 1, near "||=" syntax error at -e line 1, near "unless >"

As my brother said above: "Change to single quotes for Linux", i.e.:

perl -ne '$first ||=$_; $last=$_; $count++}{die qq(ERROR found $count 
+lines. Need >5) unless $count>5; print qq(FIRST:$first\nLAST:$last\n)
+' YOUR-FILE-NAME-here.txt
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This is to keep your shell from messing with the script by interpreting special characters.

Wow, now the perlsecret is out.