s/PATTERN/REPLACEMENT/

i am sorry for not being clear if it is
XYZATGCAATGGCGTC.....
YAZXSATGCAVVGBHYN...
there are many patterns

The sequence should be trimmed from the begining ofATGC to full sting and should give only XYZ (for example if the string XYZATGCAATGGCGTC.....

I'm not sure you've given enough information. We need a broader selection of example inputs and outputs.

For instance, here's a perfectly valid script that does exactly what you demand -- no more, no less:

#!/usr/bin/perl
#       xyzatgc.pl
#       =  Copyright 2011 Xiong Changnian <xiong@cpan.org>   =
#       = Free Software = Artistic License 2.0 = NO WARRANTY =

use 5.014002;
use strict;
use warnings;

#~ use Devel::Comments '#####', ({ -file => 'debug.log' });

#---------------------------------------------------------------------
+-------#

# Pass input filename on command line: $ xyzatgc.pl infile.txt
my $in_filename     = shift;

# Construct output filename: infile.txt => infile.out
$in_filename       =~ /([^.]+)\.txt/;
my $out_filename    = $1 . q|.out|;

# Slurp in entire input file.
my $indata          ;
{
    open my $in_fh, '<', $in_filename
        or die "Couldn't open $in_filename for reading";
    local $/        = undef;            # slurp
    $indata         = <$in_fh>;
    close $in_fh 
        or die "Couldn't close $in_filename";
};

# Substitute as required.
$indata     =~ s/XYZATGC/XYZ/g;

# Write results to output file.
open my $out_fh, '>', $out_filename
    or die "Couldn't open $out_filename for writing";
say {$out_fh} $indata;
close $out_fh 
    or die "Couldn't close $out_filename";

# Terminate.
say 'Done.';

__END__
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Input:

XYZATGC XYZATGC XYZATGC XYZATGC XYZATGC 
XYZ xyz foo ATGC atgc JAPHATGC
XYZATGC 
XYZATGCXYZATGCXYZATGC
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Output:

XYZ XYZ XYZ XYZ XYZ 
XYZ xyz foo ATGC atgc JAPHATGC
XYZ 
XYZXYZXYZ
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Now I'm going to wager that's not quite what you want. Please don't try to explain in English words what you'd rather see. Instead, show us a fuller example of input and output.

We'll see what we can do.

hi this is not wat i wanted.
$indata =~ s/XYZATGC/XYZ/g; i dont knw what string will be there after ATGC. in a string like this XYZATGCCVFGBGVFCD... as soon as ATGC is found at a particular position it should trim the entire part (including ATGC and the rest that follows it and show only the string which is XYZ and write it to new file. am i clear now


here are some exammples
input

XYZATGCACGTGFVGFCCV.......
YZXCVFDCXZATGCXCCXZZSDD
output
XYZ new file1.txt
YZXCVFDCXZ new file2.txt

- # Substitute as required.
- $indata     =~ s/XYZATGC/XYZ/g;
+ # This is not terribly efficient.
+ my @outdata     = split q|ATGC|, $indata;
+ $indata         = $outdata[0];
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I'm not the guy you kill, I'm the guy you buy. —Michael Clayton

You could try something like this

use v5.14;

use warnings;

my @values = ( qw/XYZATGCACGTGFVGFCCV
YZXCVFDCXZATGCXCCXZZSDD/);


for my $v (@values) {

  my ($pre) = $v =~ m/^(.*?)ATGC/;

  say $pre;
}
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Ya thank you every one ill try them