I was trying to extract the following words from input file.

use strict;
use warnings;

while ( my $line = <DATA> ) {

#-- would print lines starting with either 'f' or 'b':
#  if ( $line =~ /^(b|f)/) {
#    print $line;
#  }

  #-- not elegant, like the spec...
  if ( $line =~ /^(b)=(\S+)/ or $line =~ /^(f)\s+(\S+)\s+(\S+)$/ ) {
    print "$1 = '$2' (", $3 // '-' , ")\n";
  }
}


__DATA__
Input file : (to extract for b and f)
b=23 y=0x11 arg=0x70 def=0x1 val=0x234
checking system

b=71 y=0x35 arg=0x87 def=0x3 val=0x76d
h=reg.k2.io.chk 0x2001

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
f chk.fin.reg.m_cwr 0x213

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
checking system

b=40 y=0x90 arg=0x34 def=0x5 val=0x2197
f ref.grf.pin.clk_trg 0x0021
[download]

Result:

b = '23' (-)
b = '71' (-)
b = '54' (-)
f = 'chk.fin.reg.m_cwr' (0x213)
b = '54' (-)
b = '40' (-)
f = 'ref.grf.pin.clk_trg' (0x0021)
[download]

I will need to get whole sentence of b and f and ignore b that are not related to f.

Expected output :

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
f chk.fin.reg.m_cwr 0x213

b=40 y=0x90 arg=0x34 def=0x5 val=0x2197
f ref.grf.pin.clk_trg 0x0021
[download]

from the b sentence, I will need to get y, arg, def. from f sentence (naming - chk.fin.reg.m_cwr, ref.grf.pin.clk_trg)

 
Ex: 
From b :
y = 0x23
arg = 0x78
def = 0x2

From f : 
chk.fin.reg.m_cwr
[download]

You could just modify my previous code to also return the $record from get_record() and print the $record if $f ne '';

Another common technique for parsing a record like this is to return a hash (or hash ref) in case you want to access these individual parameters. The format X=3 is easy to parse, below I show how, with a more extensive code modification. There are of course other formulations of how to do this. I just went along the lines of my original post.

The idea of these Monk posts is to give you some ideas and get you "unstuck". Some effort on the OP is required to understand the technique(s) and adapt it to your specific application. Hope this helps.

Update:
I thought better of the ending condition for the main loop and changed it to end on a blank record. Also as a general point, I typically try to isolate the record parsing into a subroutine. I just made a new code release of a project I've been working on for the past year. During that time, the input format and also how the data is presented of one key file has changed 4 times! I yell at these guys when they do that, but alas, I must adapt. I derive the same info from all 4 formats, but this is not just different formats, but also different algorithms - "data" is not "information". A mess.

#!/usr/bin/perl
use warnings;
use strict;

my ($record,%hash);
while ( ($record, %hash)=get_record() and $record ne '' )
{
   next unless $hash{f} ne '';
   
   print "$record\n";
   
   print "Extra parsed info:\n";
   foreach my $key (keys %hash)
   {
      print "  $key \tvalue=$hash{$key}\n";
   }
   print "\n";
}


sub get_record
{
  my $line;
  my $record='';
  
  while (defined($line=<DATA>) and $line !~ /^\s*$/)
  {
     $record .= $line
  }
  
  my %hash = $record =~ /(\w+)=(\w+)/g;
  ($hash{f}) = $record =~ /f (.*)/;
  $hash{f} //="";
  
  return ($record, %hash);
}

=Prints:
b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
f chk.fin.reg.m_cwr 0x213

Extra parsed info:
  def   value=0x2
  val   value=0x65b
  f     value=chk.fin.reg.m_cwr 0x213
  b     value=54
  y     value=0x23
  arg   value=0x78

b=40 y=0x90 arg=0x34 def=0x5 val=0x2197
f ref.grf.pin.clk_trg 0x0021

Extra parsed info:
  arg   value=0x34
  y     value=0x90
  f     value=ref.grf.pin.clk_trg 0x0021
  b     value=40
  def   value=0x5
  val   value=0x2197
=cut


__DATA__
b=23 y=0x11 arg=0x70 def=0x1 val=0x234
checking system

b=71 y=0x35 arg=0x87 def=0x3 val=0x76d
h=reg.k2.io.chk 0x2001

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
f chk.fin.reg.m_cwr 0x213

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
checking system

b=40 y=0x90 arg=0x34 def=0x5 val=0x2197
f ref.grf.pin.clk_trg 0x0021
[download]

edit: Maybe the data is like this?

b=23 y=0x11 arg=0x70 def=0x1 val=0x234 checking system 
b=71 y=0x35 arg=0x87 def=0x3 val=0x76d h=reg.k2.io.chk 0x2001 
b=54 y=0x23 arg=0x78 def=0x2 val=0x65b f chk.fin.reg.m_cwr 0x213 
b=54 y=0x23 arg=0x78 def=0x2 val=0x65b checking system 
b=40 y=0x90 arg=0x34 def=0x5 val=0x2197 f ref.grf.pin.clk_trg 0x0021
[download]

###### update
Ok, assuming the formatting by Athanasius++ is correct.
This is a simple albeit very wordy algorithm - make a single line out of the multi-line record and run a regex on that.

#!/usr/bin/perl
use warnings;
use strict;

my ($b,$f);
while ( ($b, $f)=get_record(), defined ($b) )
{
  print "b=\'$b\' f=\'$f\'\n";
}


sub get_record
{
  my $line;
  my $record='';
  
  while (defined($line=<DATA>) and $line !~ /^\s*$/)
  {
     $record .= $line
  }
  my ($b)= $record =~ /b\=(\w+)/;
  my ($f)= $record =~ /f (.*)/;
  $f//="";
  return ($b,$f);
}

=Prints:
b='23' f=''
b='71' f=''
b='54' f='chk.fin.reg.m_cwr 0x213'
b='54' f=''
b='40' f='ref.grf.pin.clk_trg 0x0021'
=cut


__DATA__
b=23 y=0x11 arg=0x70 def=0x1 val=0x234
checking system

b=71 y=0x35 arg=0x87 def=0x3 val=0x76d
h=reg.k2.io.chk 0x2001

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
f chk.fin.reg.m_cwr 0x213

b=54 y=0x23 arg=0x78 def=0x2 val=0x65b
checking system

b=40 y=0x90 arg=0x34 def=0x5 val=0x2197
f ref.grf.pin.clk_trg 0x0021
[download]

In addition to not clearly showing your code and data, you do not show the output you want to get from the given data, you do not show the output you actually get, and you don't say how what you get differs from what you want. Please see Short, Self-Contained, Correct Example. Please also see How do I change/delete my post?