This will print the red line line and the one following:

while (<DATA>) {
  print if /red line/..././;
}
__DATA__
A red line is found 
at location 2.5 4.5 
A Blue line is found 
at location 2.6 4.5 
A red line is found 
at location 2.67 4.56 
The red arc is found 
at location 2.5 4.5
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and if you don't want the red line line, make it

  print if (/red line/..././) =~ /E/;
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