while (<IN>) {  #read file of lines of data 
   $_ =~ s/\bthree\b/3/gi; 
   $_ =~ s/\btwo\b/2/gi; 
   $_ =~ s/\bone\b/1/gi; 
   print OUT $_; # assuming we are creating a new file with the modified lines 
}

use strict; 
use warnings; 
my %number = ( 
    one   => 1, 
    two   => 2, 
    three => 3, 
); 
 
while (my $line = <DATA>) { 
    $line =~ s/(:?one|two|three)/$number{lc $1}/egi; 
    print $line, "\n"; 
} 
__DATA__ 
John has three apples, two cats and one girlfriend

John has 3 apples, 2 cats and 1 girlfriend