The following may get you started:

use strict;
use warnings;

open TEST, '>', 'test1.txt';
print TEST <<DATA;
1
2
3
5
7
9
DATA
close TEST;

open TEST, '>', 'test2.txt';
print TEST <<DATA;
2    
4
5
8
9
DATA
close TEST;

open IN1, '<', 'test1.txt';
open IN2, '<', 'test2.txt';

my $in1Line = <IN1>;
my $in2Line = <IN2>;

while (defined $in1Line or defined $in2Line) {
    if (! defined $in2Line or $in1Line < $in2Line) {
        print "No match from test1.txt: $in1Line";
        $in1Line = <IN1>;
        
    } elsif (! defined $in1Line or $in2Line < $in1Line) {
        print "No match from test2.txt: $in2Line";
        $in2Line = <IN2>;
        
    } else {
        # match
        print "Match: $in1Line";
        $in1Line = <IN1>;
        $in2Line = <IN2>;
    }
}

close IN1;
close IN2;
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Prints:

No match from test1.txt: 1
Match: 2
No match from test1.txt: 3
No match from test2.txt: 4
Match: 5
No match from test1.txt: 7
No match from test2.txt: 8
Match: 9
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