#! perl
use strict;
use warnings;
use Capture::Tiny 'tee_merged';

my $outfile = 'sqr.txt';
my $stdout  =  tee_merged \□

open(my $fh, '>', $outfile)
    or die "Cannot open file '$outfile' for writing: $!";

print $fh $stdout;

close $fh
    or die "Cannot close file '$outfile': $!";

sub square
{
    print "Enter a number (0 to quit): ";
    my $n = <STDIN>;
    chomp $n;

    while ($n)
    {
        printf "%f squared is %f\n", $n, $n * $n;
        print  "Enter a number (0 to quit): ";
        $n = <STDIN>;
        chomp $n;
    }

    print "Bye!\n";
}
[download]

Console output:

16:51 >perl 1112_SoPW.pl
Enter a number (0 to quit): 57.6
57.600000 squared is 3317.760000
Enter a number (0 to quit): .00211
0.002110 squared is 0.000004
Enter a number (0 to quit): 12
12.000000 squared is 144.000000
Enter a number (0 to quit): 0
Bye!

16:52 >
[download]

Text file output:

Enter a number (0 to quit): 57.600000 squared is 3317.760000
Enter a number (0 to quit): 0.002110 squared is 0.000004
Enter a number (0 to quit): 12.000000 squared is 144.000000
Enter a number (0 to quit): Bye!
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Note that tee_merged captures output to STDOUT and STDERR, but not input from STDIN (i.e. via the keyboard).