xa1 b t g e1
xa1 a s f a1

##</code><code>##

while (($nxt = readline($fh)) =~ /^\+/) {

##</code><code>##

.subckt a1 x y z
   xa a b c1
   xb c d e1
   xc f g h1
 .ends

 .subckt c1 x y z
  xa a b f
  xb c d e
  xc f g h
.ends

.subckt e1 x y z
  xa a b c1
  xb c d k1
  xc f g h1
.ends

xa1 b t g e1
xa1 a s f a1

##</code><code>##

use strict;
use warnings;

open my $fh, '<', $ARGV[0]
    or die "Cannot open file '$ARGV[0]' for reading: $!";

sub getsub
{
    my ($sub) = @_;
    print "inside sub for $sub\n";
    seek $fh, 0, 0;
    (/\.subckt $sub/ .. /\.ends/) && print while <$fh>;
}

my $line = <$fh>;

while ($line)
{
    if ($line =~ /^xa1/)
    {
        print "line found to be $line\n";
        my $next;
        $line =  $next while !eof && ($next = <$fh>) =~ /^\+/;
        $line =~ s/\s+$//;
        my $sub = (split /\s+/, $line)[-1];
        print "subcircuit found is $sub in $line\n";

        my $file_pos = tell $fh;
        getsub($sub);
        seek $fh, $file_pos, 0;

        $line = $next;
    }
    else
    {
        $line = <$fh>;
    }
}

close $fh
    or die "Cannot close file '$ARGV[0]': $!";

##</code><code>##

13:20 >perl 1032a_SoPW.pl subs.txt
line found to be xa1 b t g e1

subcircuit found is e1 in xa1 b t g e1
inside sub for e1
.subckt e1 x y z
  xa a b c1
  xb c d k1
  xc f g h1
.ends
line found to be xa1 a s f a1

subcircuit found is a1 in xa1 a s f a1
inside sub for a1
.subckt a1 x y z
   xa a b c1
   xb c d e1
   xc f g h1
 .ends

13:20 >

##</code><code>##

use strict;

##</code><code>##

#!/usr/bin/perl -w
...
use warnings;