If you need to slurp a big file and use minimum memory, slurp it as a string:

open FILE, '<', ....;
my $bigstring;
do{ local $/; $bigstring = <FILE>; };

## NOTE: not my $bigstring = do{ local $/; <FILE> }; This consumes dou
+ble the memory of the above.
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You can then easily process the file line-by-line, by opening the big string as a memory file:

open RAMFILE, '<', \$bigstring;
while( <RAMFILE> {
    ### process in the normal way.
}
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However, if your long running processing needs access to the lines as an array, then that could be a very inconvenient form for your task.

You might be tempted to build an index into the bigstring something like this:

my( $p, @index ) = 0;

$index[ ++$p ] = tell( RAMFILE ) while <RAMFILE>;

## Then to randomly access line $n of the file

my $nthLine = substr( $bigstring, $index[ $n ], $index[ $n+1 ] - $inde
+x[ $n ] );
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The problem is that the index will occupy almost as much memory as your original array of lines, and you're not just back to square one, but worse off.

However, you can build an index that occupies far less space:

my( $p, $index ) = ( 0, "\0" );
vec( $index, ++$p, 32 ) = tell( RAMFILE ) while <RAMFILE>;

## then to randomly access line $n of the file

my $nthLine = substr( $bigstring, vec( $index, $n, 32 ), vec( $index, 
+$n+1, 32 ) - vec( $index, $n, 32 ) );
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Putting it all together:

#! perl -slw
use strict;

my $bigstring;
do{ local $/; $bigstring = <> };
close ARGV; print length $bigstring;

open RAMFILE, '<', \$bigstring or die $!;

my( $p, $index ) = ( 0, chr(0) x ( 4 * 10e7 ) );
vec( $index, ++$p, 32 ) = tell( RAMFILE ) while <RAMFILE>;

## print the 500,000th line of the file

my $n = 500,000;
print substr( $bigstring, vec( $index, $n, 32 ), vec( $index, $n+1, 32
+ ) - vec( $index, $n, 32 ) );

<STDIN>; ## pause to check memory size

__END__
[16:02:50.49] C:\test>dir test.dat
 Volume in drive C is Local Disk
 Volume Serial Number is 8C78-4B42

 Directory of C:\test

15/09/2016  23:47     1,020,000,000 test.dat
               1 File(s)  1,020,000,000 bytes
               0 Dir(s)  379,695,529,984 bytes free

[16:02:54.35] C:\test>wc -l test.dat
10000000 test.dat

[16:03:00.02] C:\test>head test.dat
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+""""""""""""""""""""""""""""""
######################################################################
+##############################
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
+$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
+&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+''''''''''''''''''''''''''''''
((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((
+((((((((((((((((((((((((((((((
))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))
+))))))))))))))))))))))))))))))
**********************************************************************
+******************************

[16:03:03.22] C:\test>1171361 test.dat
1010000000
5555555555555555555555555555555555555555555555555555555555555555555555
+555555555555555555555555555555

# 1,774MB

[16:03:22.93] C:\test>
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