See Tutorials: Context in Perl: Context tutorial

split, in scalar context, returns the number of fields found, so you need to use list context

See also autodie and Modern Perl: the free book

#!/usr/bin/perl --
#~ 2011-10-19-05:27:59PDT by Anonymous Monk
#~ perltidy -csc -otr -opr -ce -nibc -i=4     
use strict;
use warnings;
use autodie; # dies if open/close... fail

Main( @ARGV );
exit( 0 );

sub Main {
    if ( @_ == 2 ) {
        NotDemoMeaningfulName(@_);
    } else {
        Demo();
        print '#' x 33 ,"\n", Usage();
    }
} ## end sub Main

sub NotDemoMeaningfulName {
    my( $transaction, $res ) = @_;
    open my($in), '<', $transaction;
    open my($re), '>', $res;
    while( <$in> ){
        my( $left, $right ) = split /T/, $_, 2;
        print $re $left;
    }
    close $in;
    close $re;
} ## end sub NotDemoMeaningfulName 

sub Usage {
    <<"__USAGE__";


$0    
$0    dataFile     newDataFile
    
    perl ${\__FILE__}    
    perl ${\__FILE__}    dataFile     newDataFile


__USAGE__
} ## end sub Usage


sub Demo {
    my ( $Input, $WantedOutput )  = DemoData();
    NotDemoMeaningfulName( \$Input, \my $Output );
    require Test::More;
    Test::More::is(
        $Output,
        $WantedOutput,
        ' NotDemoMeaningfulName Works Aas Designed'
    );
    Test::More::done_testing();
} ## end sub Demo

sub DemoData {
    my $One = <<'__One__';
2010-12-01T21:15:40.000+01:00
2010-12-01T23:28:04.000+01:00
__One__

    my $Two = <<'__Two__';
2010-12-01
2010-12-01
__Two__

    return $One, $Two;
} ## end sub DemoData

__END__
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