Will handle arbitrarily large files with minimal memory use:

use strict;

open my $FILE1, '<', 'my/input/file1' or die "Could not open FILE1: $!
+";
open my $FILE2, '<', 'my/input/file2' or die "Could not open FILE2: $!
+";
open my $FILE3, '<', 'my/input/file3' or die "Could not open FILE3: $!
+";;

open my $FILE4, '>', 'my/output/file' or die "Could not open FILE4: $!
+";

while (my $file1 = <$FILE1>) {
    my $file2 = <$FILE2>;
    my $file3 = <$FILE3>;
    
    chomp $file1;
    chomp $file2;
    
    print $FILE4, "$file1 $file2 $file3" or die "Could not print to FI
+LE4";
}
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Update: We assume that each file has the same number of lines.