Hello sar123, and welcome to the Monastery!

By the time the conditional:

if ($_ =~ /^xa1/) {
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becomes true in the main while loop, the $fh file pointer has already been moved past the part of the data file you want getsub() to read. You will have to reset it using seek. But be sure to return it to its former position before exiting the subroutine, or you will enter an infinite loop! Best to make it local within getsub, like this (untested):

sub getsub
{
    my $sub = $_[0];
    print "sub entered for $sub\n";
    local $fh;
    seek  $fh, 0, 0;

    while (<$fh>)
    {
        print if /\.subckt $sub/ .. /\.ends/;
    }
}
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Update 1: Turns out that local doesn’t work the way I hoped it would. :-( This works:

use strict;
use warnings;

open(my $fh, '<', $ARGV[0]) or die "Could not open file: $!";

sub getsub
{
    my $sub = $_[0];
    print "sub entered for $sub\n";

    open my $fh2, '<', 'subs.txt' or die "Could not open file: $!";

    while (<$fh2>)
    {
        print if /\.subckt $sub/ .. /\.ends/;
    }

    close $fh2 or die "Could not close file: $!";
}

while (my $line = <$fh>)
{
    if ($line =~ /^xa1/)
    {
        print "line found to be $line\n";

        while ((my $nxt = readline($fh)) =~ /^\+/)
        {
            $line = $nxt;
            print "line changed to $line\n";
        }

        $line =~ s/\s+$//;
        print  "last line is $line\n";
        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;
    }
}

close $fh or die "Could not close file: $!";
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Output:

 2:55 >perl 1032_SoPW.pl subs.txt
line found to be xa1 a s f a1

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

 2:56 >
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Update 2: To eliminate the warning that arises when the line beginning xa1 is the last line in the data file, change:

while ((my $nxt = readline($fh)) =~ /^\+/)
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to:

while (!eof && (my $nxt = readline($fh)) =~ /^\+/)
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Hope that helps,