# create a variable to store the output in
my $output = "";
# open a "file" that will store output in the variable
# (requires 5.8; before that use CPAN module IO::Scalar)
open my $fh, ">", \$output or die "Unexpected open to scalar failure: $!";
# save the default output file handle
my $select_save = select($fh);
# call your function
call_function();
# clean up
select($select_save);
close $fh;

# use output
print "Got output: $output\n";