... open(my $READER,'+<',$Filename_S) or die "Can't open '$Filename_S'! $!"; open(my $WRITER,'+<',$Filename_S) # Don't use '+>' or die "Can't open '$Filename_S'! $!"; # The overflow buffer: my @Buffer_a; while (<$READER>) { # Read (past tense) a line - buffer its replacement ... # Modify the line here! my $Update_s=... push(@Buffer_a,$Update_s); # Write from the overflow buffer if we can ... while (@Buffer_a && length($Buffer_a[0]) < tell($READER)-tell($WRITER)) { # Enough room to write $Buffer_a[0] so write it ... print $WRITER shift(@Buffer_a); }; }; # Nothing more to read ... close($READER) or die "Can't close '$Filename_S'! $!"; # If there's anything in the buffer write it ... while (@Buffer_a) { print $WRITER shift(@Buffer_a); }; # Truncate the file, in case, what we're writing is shorter than what we read truncate($WRITER,tell($WRITER)); # ... and close close($WRITER) or die "Can't close '$Filename_S'! $!"; ...