use warnings 'all';
use strict;
use autodie;

open my $input, '<', 'file1.txt';
open my $input2, '<', 'file2.txt';
open my $out,   '>', 'output.txt';

while ( my $sentence = <$input> ) {

    my $substring = '\.c.*?entry';
    $sentence =~ s{$substring}{$file2};

    print $out , $sentence;
}