In the open doc there is an example:

close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
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You could combine that with local *STDOUT to limit that capturing to a scope:

#!/usr/bin/perl
use warnings;
use strict;

sub writes_to_stdout {
        print "some information\n";
}

sub captured {
        local *STDOUT;
        my $result;
        open STDOUT, '>', \$result;
        writes_to_stdout();
        return $result;
}

my $val = captured();
print "No output yet\n";

print "But now: $val";

## and this is what it writes:
$ perl foo.pl
No output yet
But now: some information
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Update: added complete example