Similar to AnomalousMonk's solution but using look-behind and -ahead assertions and tr with the "complement" flag.

use strict;
use warnings;

open my $logFH, q{<}, \ <<EOD or die qq{open: << HEREDOC: $!\n};
2009/06/09 10:11:12 read error: file="/path/to/file" reason: gone
2009/06/09 12:43:27 created: file="/path/to/file"
EOD

while( <$logFH> )
{
    s{(?<= file=" ) ( [^"]+ ) (?= " )}
     {do { my $cap = $1; $cap =~ tr{/}{x}c; $cap }}xeg;
    print;
}
[download]

The output.

2009/06/09 10:11:12 read error: file="/xxxx/xx/xxxx" reason: gone
2009/06/09 12:43:27 created: file="/xxxx/xx/xxxx"
[download]

I hope this is of interest.