use strict;
use warnings;

my $inp = "source.cbl";
my $out = "source.ada";

open INP, '<', $inp or die "Can't open $inp: $!";
open OUT, '>', $out or die "Can't open $out: $!";

while (<INP>) {
    s/WORKING -STORAGE SECTION\./declare/;
    s/PROGRAM -BEGIN\./begin/;
    s/DISPLAY/PUT/;
    s/ACCEPT/GET/;
    s/PROGRAM -DONE\./END;/;
    print OUT;
}

close INP;  # no worries for closing input file
close OUT or die "Can't close $out: $!";