If your using *nix, instead of using fork, use my $pid = open( KID, '-|') or die $!; to kick of the child process. The child can then print __LINE__, "\n"; at strategic points (or every line if you wish) and the parent reads from <STDIN> (Updated: Thanks runrig) <KID> to get the line numbers.

See the "Safe pipe opens" section of perlipc for examples.

I couldn't get this to work under Win32 and tried pipe as an alternative, which works except I cannot work out how to get the parent process to detect that the child has closed the pipe? Maybe someone else knows the trick for this?

#! perl -sw
use strict;
use IO::Handle;

pipe(FROM_CHILD,  TO_PARENT) or die $!;
TO_PARENT->autoflush(1);

if( my $pid = fork ) { #parent
    while( <FROM_CHILD> ) {
        chomp;
        print "\rChild is processing line:$_ ";
    }
    print 'Child appears to be finished.'; ## This never reached?
    close FROM_CHILD;
}
else {
    for (1..10) {
        print TO_PARENT  __LINE__, $/; select undef, undef, undef, 0.0
+5;
        print TO_PARENT  __LINE__, $/; select undef, undef, undef, 0.0
+5;
        print TO_PARENT  __LINE__, $/; select undef, undef, undef, 0.0
+5;
        print TO_PARENT  __LINE__, $/; select undef, undef, undef, 0.0
+5;
    }
    close TO_PARENT;
}
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