I'm not sure why you are using IO::File when three-argument open anf lexical filehandle will serve as well. I have used your data but have added a bogus line to demonstrate how to handle lines that don't fit your expectations. I use a regex capture to get the variable name (captured in $1) and print it out; obviously, you can assign it to a variable for later use. Once you know what a line is and have dealt with it, use next to get the next line rather than testing further. Here is the input file

.rem *****************************************************************
+*****
.rem *                                                                
+    *
.rem *                           deb123.rpt                           
+    *
.rem *****************************************************************
+*****
.rem
.rem ------------------------ book_tab variables ---------------------
.rem
.declare book_no                     a14
.declare l_code                      a10
.burble nonsense directive here
.declare l_book                         a12
.declare book_date                     a13
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the code

use strict;
use warnings;

my $inFile = q{spw623147.txt};
open my $inFH, q{<}, $inFile
   or die qq{open: $inFile: $!\n};

while ( <$inFH> )
{
    if ( m{\A\.rem} )
    {
        print qq{  Comment Line: $_};
        next;
    }
    if ( m{\A\.declare\s+(\S+)} )
    {
        print qq{     Data Line: $_};
        print qq{   Variable is: $1\n};
        next;
    }
    print qq{Not recognised: $_};
}

close $inFH
   or die qq{close: $inFile: $!\n};
[download]

and the output

  Comment Line: .rem *************************************************
+*********************
  Comment Line: .rem *                                                
+                    *
  Comment Line: .rem *                           deb123.rpt           
+                    *
  Comment Line: .rem *************************************************
+*********************
  Comment Line: .rem
  Comment Line: .rem ------------------------ book_tab variables -----
+----------------
  Comment Line: .rem
     Data Line: .declare book_no                     a14
   Variable is: book_no
     Data Line: .declare l_code                      a10
   Variable is: l_code
Not recognised: .burble nonsense directive here
     Data Line: .declare l_book                         a12
   Variable is: l_book
     Data Line: .declare book_date                     a13
   Variable is: book_date
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I hope thisis of use.

Cheers,

JohnGG

Update: Fixed typo