Perl Flip Flop operator and line numbers -

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i noticed while looking @ another question...

if have script this:

while (<>) {   print if 5 .. undef; }

it skips lines 1..4 prints rest of file. if try this:

my $start_line = 5; while (<>) {   print if $start_line .. undef; }

it prints line 1. can explain why?

actually i'm not sure why first 1 works.

hmmm looking further found works:

my $start = 5; while (<>) {   print if $. ==  $start .. undef; }

so first version magically uses $. line number. don't know why fails variable.

the use of bare number in flip-flop treated test against line count variable, $.. perldoc perlop:

if either operand of scalar ".." constant expression, operand considered true if equal (==) current input line number (the $. variable).

so

print if 5 .. undef;

is "shorthand" for:

print if $. == 5 .. undef;

the same not true scalar variable not constant expression. why not tested against $..


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