my $pattern = $field[0];  
my $replacement = $field[1];  
  
my  $test = $field[0];  
  
if ($test =~ /^$pattern$/) {  
 
# $& holds the entire regex match. 
    my $test1 =~ s/$&/$replacement/; 
 
# Or you could use $1 if you would like.    
    my $test2 =~ s/$1/$replacement/;  
 
    print $test1, "\n"; 
    print $test2, "\n";  
 
} else {  
    print "Did not find a match for $pattern in $test";  
}