SWF::BinStream::File allows writing to a file handle, so use IO::Scalar to create a file handle that writes to a string and then you can print the string.

I've not tested this code but something along these lines may do what you want.

use SWF::File;
use IO::Scalar;

my $swf_data = '';
my $swf_fh = IO::Scalar->new( \$swf_data );
my $swf = SWF::BinStream::Write->new($swf_fh);
...
$swf->close();
close $swf_fh;
print $swf_data;
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