Something like this, for example (UNTESTED):

my $dir = '/some/path/to/files';

local @ARGV = glob "$dir/*";  # get all files in $dir

local $^I = '.bak';  # modify files "in-place"
                     # non-empty string required for Windows

while ( <> ) {  # loop through all files

    s/regular expression/replacement text/g;  # make modification

    print;  # save the result

    }
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