Assuming you're ok with loading the entire file into memory, you can use the following:

$str =~ s{$pat}{
   prompt( $_, $-[0], $+[0]-$-[0], $&, $repl, $`, $' )
   ? $repl
   : $&
}eg
[download]

or

use String::Substitution qw( interpolate_match_vars last_match_vars );

$str =~ s{$pat}{
   my $true_repl = interpolate_match_vars( $repl, last_match_vars() );
   prompt( $_, $-[0], $+[0]-$-[0], $&, $true_repl, $`, $' )
   ? $true_repl
   : $&
}eg
[download]

The latter supports interpolation of numerical captures such as $1 and ${1} in $repl, though you might need to escape some dollar signs and backslashes.

It's up to you to provide $str (the contents of the file), $pat (the pattern for which to search as a string or compiled regex), $repl (the replacement string) and prompt.

Args passed to prompt:

• $str
• The position in $str at which the match starts.
• The length of the match.
• The substring of $str that matched.
• The string with which the match will be replaced if prompt returns a true value.
• The substring of $str that precedes the match. Use however much of it that you want.
• The substring of $str that follows the match. Use however much of it that you want.

Have prompt return a true value to replace, or a false value to skip the replacement.

Update: Added second version and a lot of info.

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