open( DAT, '<', 'export.dat' )
    or die "Open for read failed on export.dat: $!";
my $tmp;  # file handle

while ( <DAT> ) {
    next unless ( /\S/ );
    if ( /^1/ ) {
        if ( defined( $tmp )) {
            output_data( $tmp );
        }
        open( $tmp, '+>', 'export.tmp' )  # open read/write, truncate first
            or die "Open failed for export.tmp: $!";
    }
    print $tmp $_;
}
output_data( $tmp );

sub output_data
{
    my $fh = shift;
    seek( $fh, 0, 0 );  # rewind to start of file
    # do stuff with export.tmp contents...
    close $fh;
}