use strict;

my @lines;

# read the whole file into an array
{ # we use a block here so the lexical filehandle variable goes automatically out of scope and gets closed
    open my $filehandle, '<', 'path/to/my/file';
    @lines = <$filehandle>;
}

# do something with the lines in @lines

#now save the array to disk
{ # we use a block here so the lexical filehandle variable goes automatically out of scope and gets closed
    open my $filehandle, '>', 'path/to/my/file';
    print $filehandle @lines;
}